My .02cents/ experience

After many months of running perfectly on a new motor on my 86' 4runner , it suddenly developed symptoms such as you describe and seems to be common to these engines. I checked vac lines , vsv's, wires, grounds, fuel presuures , pumps and sensors and the propblem just slowly kept getting worse - to the point where I was constantly adjusting the Idle just to keep it running.

One day while troubleshooting the A/C which would sometimes idle up and sometimes not and other times surge back and forth , I pulled the adjustable idle -up vacuum valve that goes to the A/C vsv and the power steering . I couldn't find any directions in the manual to test it so I huffed and puffed in and out of all the different ports and it just didn't seem like it was really working properly. I pulled the round metal cap of the top of the Vac diaphram and inspected the material and found it had a very small tear. Too long a story short, I replaced the 22 year old valve (I think around $23.00) and it cured not only my A/C problem but my idle issue also