irratic idle

I've got a 1988 Ford F-150 XLT with 302. It idles irratically at all times. At a stop, it will idle up to 3000 rpms, then drop down to 600, then back up again. While driving, when push clutch in to change gears, it will rev up to around 3000 rpms. I have done complete tune up and changed the idle control valve. Still doing the same thing. Any suggestions?

Air (vacuum) leak.

Try a throttle position senser. I had a Ford Tempo that did the same thing.