Just got an 87 350tpi IROC. Sometimes while driving through town, when I stop at a red light it starts to jump up and down from 1,000rpm+- to 400rpm+- and unless I get on the gas it will die. It normally idles at 600+ just fine. It runs at less than 220 F* most of the time but in heavy traffic it will go just above.

Only does this once in a while, not everyday.

 

Check your IAC (there's an article on it in the tech articles section on the site), also, check your fuel pressure, just in case. An injector may be leaking or your fuel pressure regulator may have small tears in it (your same symptoms happened to me, and it got alot worse very quickly)