What check valve are you useing? if its not the original YS get one and try it. If it is the YS type make sure theres no debries in it and its working correctly. I know you said it was holding good pressure ,but a tiny bit of grit can cause alot of problems at times.
There was some issues with the 91's piston ring not sealing good, thus the gas porting come about to add more pressure to the ring as it come up to TDC to fire, the gas porting solved the problem and its not hard to do. YS did my 91 FZ for free. Theres some detailed threads by Dave Shadel on how its done.
Which engine do you have the FZ or AC?
Good Luck let us know how you make out.