I have an issue with trans to flex plate

I would double check the bell housing bolts for tightness. From what was typed- I feel like the ring gear is out of alignment with the starter- bell housing alignment or double check the ring gear mount on the torque converter. The other issue I had with a 904 that was commercially rebuilt was the pump bushing was not installed (may not be your case) but the converter started to oblong and the starter would grab and go. Sometimes binding up. Again just a thought. If mine I would be pulling the transmission and checking crank runout to start. Once apart I believe you will find the start of your answer.
Good luck!
Syleng1