87 300E Idle problem

[dukeofbluz]

Hi everyone, this is my 1st post.
I bought 87 300E and just love it. The other day the idle started to surge up then down. It goes from around 500rpm to 2000rpm with about 3 seconds between. I cant find any vacuum leaks. I had removed the idle control valve and cleaned it, still the same. So after standing there watching the idle surge up and down I unplugged the Idle control valve, the car started running fine, I took it for a drive and everything worked great, turned on the A/C and still ran great. Thinking the car would run fine without this valve connected I forgot about it for around 5 hours. Then I started and it would not stay running. I bought a used ( but tested ) idle control valve and installed it, nope same problem.
Any Ideas?
I have done a search but I hoped knowing the car runs great without the idle valve, maybe it would help make my diagnosis easier.
Thanks, Duke