Hi Daniel.
I had a 1991 1.7i, and sometimes my idle was high. I also had the same problem with my current 440 (2.0i). My revs died slowly on this one, and my idle was high. I actually had the rare case of a knackered injector. It cost me £40 to have one shipped from Volvo in sweden, but ive been told this is a rare case. To check its not that, remove your spark plugs one at a time (so you can remember what hole they came from)and see if any of them are black on the end. Also smell them to see if they smell of petrol. If youre not sure, clean the ends up, put them back in, start her up and leave her for a few minutes. Turn her off, check them again. If any are black, it could be you need a new injector. It will need to be replaced with a new one which you will have to go to a Volvo dealer about(if you know someone with a private garage, ask them to get it cos they can probably get it cheaper) It will be a genuine Volvo part- there are no matching injectors in any other cars. You can do it yourself in about 2 hours, but it does require taking the inlet manifold off. Tricky if youre not good with your hands!! The other solutuion could be a dirty sensor. As you look a the inlet manifold (the shiny 4 pipes with injection written on them), there is a big round black pipe that comes off and goes to the air filter. Half-way along this pipe sticking out in front of you should be a wire and a blue plug, going into the pipe. Pull this out and see if it makes a difference-it should make the revs fluxuate or go down. If it does, or even if it doesnt, take the blue plug off. Clean it and throughly dry it with some WD40 (put the WD40 on some clean rag, dont spray the stuff on to it), along with the pointy shaped black sensor. Then you will need a high pressure blast of air on the pointy bit and the blue plug to muck them out. Give them another wipe-make sure they are both dry and re-assemble. Stick it back in and start her up again. If the revs go down, sorted. If they dont, i only know one more thing. The accelerator is connected in the engine next to the inlet manifold. You can rev the car with the bonnet up by turning the it slightly and lettin it go. If you arent sure where it is, pop the bonnet and get some one to rev your car while you look to make sure you know where it is. Once youve got it, you will see the wire its attached to. If you follow the wire up a little bit, it should go through an eye and then into a thick round hard black pipe with a nut on the end. You can tighten and slacken the nut by holding one side with a spanner and turning the other. This in turn will tighten or slacken the wire. It should really be tight-it will pull quicker when you rev it therefore decreasing your revs quicker. Thats what i had to do. Try the accelerator and sensor cleaning first. If they dont work, check your plugs. If you arent sure then go to a garage. It will cost but it will save you alot of money in the long run. If it is your injector, then its possible that you are washing your bore out with petrol-this means you will be wasing all the oil out so the oil will be very thin and not much use. Get your dipstick and rub some oil on your fingers-if the oil off the stick smells like it has petrol in it, get to a garage (preferably a Volvo one) and tell them - you could blow your engine up!!
Im not saying that any of these are the solutions you are looking for, but the symptoms you have are similar to what i had, and a combibnation of all 3 sorted it out. She now goes like a rocket!! I hope i didnt sound too patronising like youre stupid or somthing-just want to make sure i directed you to the right parts incase you didnt have a clue! Im no mechanic, im actuallly a graphic design student, but i done all the aforementioned jobs with a little help from my Dad. As long as you know youre way around the engine, they shouldnt be a problem.
Good luck - post a message when you get it sorted and let me know what the problem was.

Daniel (21)