Idle drop-out on '88 3.0L V6
 

I have an '88 4Runner with a 3.0L V6. When I first start it up from cold, the idle goes up to ~1500rpm (as it should, I believe), but will intermittently drop to ~500rpm, then bounce right back to 1500rpm. It will do this for about 3-5 minutes until everything warms up, then will idle ~750rpm normally.
I've checked coolant levels, replaced MAF, spark plugs, wires, distributor cap; normal minor tune-up stuff. Vacuum hoses appear to be in good shape. Anybody have any suggestions?