1988 760 turbo with intercooler.
For some reason unknown to me or my mechanic my Volvo is going through Idle Control Valves, 2 in 4 months. When a new idle control valve is installed the car runs great for a few hundred km, it's easy to start, idles at a constant RPM, no hesitation while driving, smooth excelleration and decelleration, no signs of anything being wrong. Then the engine suddenly decides that it can no longer run at a constant RPM. It decides to go from 700 RPM to about 3500 RPM then back down again, up and down, up and down unless I tap the gas peddle lightly. Then it only reves to about 1500 RPM and back down to 700 again and as before it never levels out and I have to keep tapping the gas pedal so it does not over rev. While on the highway when the valve has decided to quit working correctly and with my foot off the accellerator pedal the car travels at about 85kmh.
I have inspected the old parts as well as I could and they both looked clean inside but I have as yet taken one apart to see if all or any parts move freely.
As my mechanic is not familiar with Vovo's and I live in an isolated part of Northern Canada with no other mechanic within hundreds of km any help in this matter would be really apreciated.
Thanks to any of you that can be of any help at all.