I have a 1995 M Reg Volvo 850 with the 2.5 5 cylinder motor. It idles erratically between 500 and 1000 RPM, it doesnt die but just constantly fluctuates between them RPM's. It only does it every once in while when it rains alot or hit a deep mud puddle, after that it idles that way for two days and then clears up like nothing happened. I was frustrated with it yesterday and cleaned up the throttle body and all the hoses to see if there were any leaks and put it all back together last night and still had the problem. Started it up this morning and no problems all day. Any help would greatly be appreciated

 

I don't know if this will help, but I'd like to offer. My car was idling really roughly, and felt like it was on the verge of dying. Took it to the shop and found out only the bosch spark plugs recommended by volvo will work for that car. It may be that your spark plugs aren't firing. Also, if you have had them changed recently the ignition wires could have put on in the wrong order. My buddy was firing in the wrong order which is terrible for the car. Basically the spark could fire when the piston is down and not fire when it needs to combust. Although this would probably be an all-the-time problem. Anywho the firing order is 1-2-4-5-3 (cylinders) looking at the engine from the front.

Hope these help. Good luck and let us know if you find the problem in case others have the problem.