It's obviously electrical and also occurs often enough to duplicate SO it can be solved by systematically diagnosing the related circuits using a OEM Volvo wiring manual for your '93 850. One needs to trace it, what does/doesn't have proper voltage when it's not starting? A good technician can fix it but finding a guy willing to spend the time might not be easy. Most flat rate technicians don't enjoy spending alot of time on one car tracing electric circuits and not all techs are good at it.
Obviously you've had several people guessing but none have fixed it. Without seeing a car like that, I won't even guess from here.

I've heard that the wiring harness's on early 850s could rub though, short just above the steering column and that it could then cause strange electrical occurances but I can't say that I've seen that myself. To check, you'ld need to remove the upper dash pad and instrument cluster to visually inspect that harness.

It will take a very knowledgeable and dedicated Volvo tech to solve a problem like that but I do believe it can be fixed, not impossible.