Let me first start by saying that I am was a trans mechanic for many years until I was just too broken to keep doing it. Here's my 2 cents. It is very strange that you are having as much problems with a manual as you are. Usually crazy problems like this are associated with automatics. It wouldn't be a bad idea to talk to a couple other specialty shops. Maybe even let them look at the car. You might learn something that the original mechanic didn't know (like the RP thing, who would have guessed - not me). But, under no circumstances should you let them take it apart to inspect; unless you are willing to pay for the rebuld again. Not that all transmission shops are shady, that is just the nature of transmissions. This is especially the case with yours since you have been having all of these odd problems. A mechanic knowing what has happened to now will be looking very closely at everything. And trust me, you can always find something that is slightly worn or damaged. Learn what you can and take it back to the first guy. He sounds willing to stand behind his work and you have already paid him. Every mechanic has had cars that are nightmares. this one is his. It is just part of the business. I suspect that if he can't fix it he will send it to another shop himself and probably on his dime too.