I'm having a bit of trouble diagnosing this problem with my '92 302. It typically only happens on warm days after the engine is up to temp. It idles fine and runs fine, but just off idle it stumbles and sometimes stalls if I don't work the pedal to keep it alive. I've just done a tuneup, that didn't change it. I pulled the IAC valve and cleaned it, that seemed to make a difference but it's hard to tell since the weather is cooler now and it has still stumbled since cleaning the valve. I've thought about a vacuum leak or TPS problems, but both of those would be constant problems rather than the intermittent one I have. I'm also wondering if it could be low fuel pressure allowing vapor lock when it's warm. Of course, it could still be the IAC valve... maybe cleaning it didn't fix it or I didn't get it clean enough. I could use some ideas.

 

Pull and clean the throttle body

 

I'll give that a shot. Looks to be a fair bit more complicated than pulling the IAC valve.