Well first of all your running to much nitro thru your MVVS unless you have installed some head shims. The MVVS line of engines where designed for no nitro because it's to expensive in europe. Contact the guys a Morris hobbies and order at least two head shims for your .61 before going any futher. Unless you switch to FAI or zero nitro fuel. You will notice a difference. And while you have them on the phone ask them how to adjust the mini pipe you have. They can be very helpfull. They have been for me.
Yor problems on the ST .91 sounds like a tank problem. I could be wrong, but it's a good place to start. Your clunk should be at least a half inch away from the back of the tank. Make sure nothing is restricted as well, i.e. pinched or blocked fuel tubes. Do you use an inline filter? If not make sure nothing is floating around in the tank to get caught in the carb. I've seen this happen a couple of times until the tank was cleaned out or a filter was used.

I'm sure you will get more places to look at from the rest of us.
Good luck and let us know what you find.