Most everything I can think of has been mentioned by others and I've gone through many of them. And they mostly all will cause some sort of code and turn on the dash light. I was wondering if your dash check engine light might be burned out, have you checked all the sockets for codes especially the A2 & A6.

My first thoughts where sensors or vacuum leak - but those should give codes. Might still be the dist cap/rotor - I've had small cracks in each at somepoint that have made for rough running. And then the plug wires - they might have gone bad - you could do the old spray with WD40 to see if that helps.

I did buy a used 245 that was about 90 degrees off in timing and it did run much better after correcting. Don't know if 850s are as tolerent that way though.

Good luck and let us know if you find the problem!