I STILL have the same problem with a 94 Pontiac SSEi, never resolved, the car is currently not being driven. Mine seems to miss more on #5 cylinder and on a few others as load is increased. It also only misses when under boost conditions. Sounds like you have done everything I had tried also. I don't remember checking the timing. If I did it wasn't off. I have tried to find a fuel pressure tester. Have been wondering if the pressure may be low.
Let me know if you get your problem resolved.