I know you want to believe it is the clunk...but listen to this. My G-26 ran great, and reliably in almost every attitude...but RPMs would sag immediately when I went into a (left wing side down) knife edge attitude. It never quit, but I didn't hold it in that position for very long either...and it sure didn't give me much confidence that it would continue to run. I was baffled as to what may be the cause. Other than that little glitch, it ran perfectly. Because of some non-related work on the plane, I had to completely retune my carb. Now, with the carb PROPERLY tuned, the sagging in knife edge has disappeared. Everything else remained the same...same plane...same fuel tank, etc etc. My suspicion is that due to improper tuning, it was becoming ever so slightly lean in knife edge, and the correct retuning of the carb solved the problem. I hope this clue works for you too.
jim