Is there any historical evidence that Paul's letters may have been tampered with?

The term "historical" is a bit vague because it could encompass the history of textual criticism as well as "higher criticism," meaning source criticism using the historico-critical method. I will briefly address them both.

As far as text criticism goes, the Pauline authorship of a the key verse, "the women should keep silence in the churches," (Corinthians 14:34) has been challenged on two grounds. 1) Codex Vaticanus continues scribal markings indicating that the passage may have been inserted. 2) the passage appears in several different places in old manuscripts. This article summarizes. The fact that the verse seems to contradict other teachings of Paul need not concern us here.

As far his historical criticism goes, 1 Corinthians is undisputed as a whole, but 1 Timothy, which forbids women to teach or otherwise hold positions of ecclesiastical authority, is thought by some to be a forgery. Among the historical bases for this is the use of the term episcopos (bishop), an office not mentioned in Paul's undisputed writings, and which is thought to have emerged several decades after this death. Also, the Pastoral epistles generally address an orthodox teaching (gnostic Docetism) that does not seem to have challenged the church when the undisputed letters were written. The Oxford University Press concludes:

Scholars generally agree that the Pastoral epistles were written by the same author. The writing style, vocabulary, general themes, and specific content are all very similar, but they are not Pauline. Over one-third of the vocabulary in these three letters is not found in any of the Pauline letters (including the Deutero-Pauline letters). The vocabulary, moreover, carries a meaning that is more consistent with second-century Christian usage than with Paul. The opponents described in these letters appear to adhere to a Gnostic Christology, a form of Christianity that did not exist during Paul’s lifetime. Finally, the Pastoral epistles presuppose a community that is hierarchically organized with bishops and deacons serving specific roles. Paul’s communities, on the other hand, were charismatic communities; that is, all members were endowed with spiritual gifts of equal importance so no one could lord it over another. The Pastoral epistles, then, reflect a church structure that developed well after Paul’s death.

Minor correction to the OP. This statement is not true: "Paul's writings were considered Scripture by his contemporaries." Paul wrote when only the OT writings were considered scripture. It would be in the next generation or two that his letters were collected and gained the status of sacred scripture.

Paul himself wrote in one of his letters that some were misrepresenting what he said. It seems that they were making verbal charges against him, twisting what he had actually taught about living morally upright lives, claiming that he was actually encouraging immoral, licentious living. This is stated in his letter to Roman Christians:

"Someone might argue, 'If my falsehood enhances God's truthfulness and so increases his glory. why am I still condemned as a sinner?' Why not say - as we are being slanderously reported as saying and as some claim that we say - 'Let us do evil that good may result?' Their condemnation is deserved." Romans 3:5-8 N.I.V. 1987 ed.(emphasis mine)

Note that Paul is here repeating what some of his accusers say when they misrepresent him. However, Paul's letter where he reports this slander has not been tampered with. It has come down to us from the middle of the first century intact, the same now as is the oldest available manuscript copy of the original.

There is another point made about difficulties understanding what Paul has written, but by another apostle, in this quote:

"Bear in mind that our Lord's patience means salvation, just as our dear brother Paul also wrote to you with the wisdom that God gave him. He writes the same way in all his letters, speaking i them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction." 2 Peter 3:15-16 (Ibid)

There may be historical examples of claims that Paul meant to write something different to what has come down to us (not that I know of any, but that's nothing to go by), but there are no copies of Pauline epistles that contain different teaching to what is in the New Testament we have today.

That is not to refute the fact that some people, somewhere along the road, have, indeed, tampered with Paul's writings. There are variant manuscripts. But the object of any such exercise would be to distort what he wrote originally. This question seems to be looking for evidence in history that what we have today in the New Testament is not entirely, authentically, what Paul initially wrote. The difficulty lies in not studying the whole body of writing Paul left us, so as to get the full measure of all of his teaching. When that is done, apparent contradictions clear up.

Finally, one historic matter that helped ensure the apostolic writings were safeguarded from any such supposed tampering, is the battle that early ensued over the threat of Gnosticism. By the middle of the second century, Gnosticism had wrapped its radically different beliefs in Christian language. The early Church response was for leaders (like Irenaeus) to counter that with soundly biblical gospel preaching, of which Paul's writings were foundational. Iranaeus's seminal work, Against Heresies, shows a thorough knowledge of both the Bible and Gnostic teachings, in order to refute the latter. Thus, apostolic teaching was protected, including Paul's teaching.