I had a similar problem on my 88 4Runner. Took me a while but I did finally discover and correct the problem.
To characterize the problem-
1. Intermittently high, but steady idle (1500-2000 rpms) for over 2 years which resolved by popping the gas pedal.
2. Initially only in summertime, but had progressed to year round.
3. Discovered if the gas was released slowly problem occurred, but if released suddenly, no problem.
4. The throttle assembly could easily (i.e., light digital pressure) be returned to normal when high idle was present.
5. Dashpot ruled out by inspection and using flat bladed screwdriver to push rod going to dashpot diaphragm down during high idle with no effect on rpms.
6. Thorough cleaning of throttle body with throttle/choke cleaner--no effect (but lots of gunk).
Decided it had to be either:
1) weak throttle valve spring--but why not problem all the time?
2) dirty throttle valve rod--why didn't cleaning affect?
3) throttle cable hanging up--made sense with 320,000 miles, sheath may contain gunk increasing friction between cable and sheath, which could be exaggerated in summer by heat expanding metal cable. Fast release of throttle would overcome gunk friction; hence, normal operation when gas pedal popped.

Removed 2 bolts holding throttle cable sheath flange to firewall, and the cable from the throttle assembly. Attached cable lubrication tool to cable end. Sprayed in lubricant until draining at firewall. To my surprise it also drained from undiscovered crack in cable sheath at sharp bend before the flange. Reassembled, took for long drive with many stops--worked fine for 3 days but is now beginning to stick again. Decided to buy a new one; unfortunately Toyota "only" part so I paid $71 from http://www.toyotapartszone.com/1988/78-51-2269-toyota-4runner-accelerator-link.html. If anyone knows of a less expensive source for Toyota "only" parts I would appreciate knowing of it.