You said you had a mail order tune right? So that means that you don't know what's been adjusted in the tune without seeing it and comparing it to a stock file.
You're really worried about it being a broken valve spring. I highly doubt that it is. If it takes the load all the way up to 4k rpm and then decides to mess up CONSISTENTLY there, then so be it. But I don't see a mechanical issue being consistent. I definitely think that it's tune related from the converter install.
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