Surging Idle.....

[fred cook]

A friend of mine has a 1987 924S that has a surge in the idle. It will idle at approximately 900 rpm for a few minutes and then increase the idle speed to about 1300 rpm for about 90 seconds and then return to normal. This repeats as long as the car is allowed to sit and idle. The fan comes on at what seems to be an appropriate temperature (new radiator thermal switch and fan relay). It doesn't seem to really be hurting anything, but is annoying. Any ideas?

Thanks,

Fred Cook