'99 idle problem driving me crazy

I have a 1999 SE Escort, single overhead cam. The idle will constantly goes up and down. Goes up for about 2 seconds, then back down for about 3 seconds. This cycles over and over. I've replaced the idle air valve twice, can't find any vacuum leaks, unplugged the AC compressor, replaced the MAF sensor........I'm completely stumped. Please help.

Just because you can't find a vacuum leak doesn't mean that there isn't one...
If you haven't done so, when the engine is running disconnect one of the hoses from the vacuum port on the intake and plug the hole. Look for changes in behavior. If none, plug it back in and disconnect another... see if you can get the engine to stop surging and you might be able to identify which direction to follow the vacuum lines.
If this doesn't lead anywhere, check into your fuel supply... plugged filter, pressure regulator, etc.

you also could spray some carb cleaner around vacc lines and if you hear a change in idle that line may need looking into. You mention that you replaced the iac two times did you notice if the gasket was in good shape and not leaking there? did you look inside the intake and see how much carbon/gum may have build up inside there and the throttle body?

Thanks for the suggestions. This is one of 4 cars I'm working on right now, but as soon as I get a chance to I'll check it out I'll post the results. Mahalo...