Trouble starting in high outdoor temp

I have a 1993 Grand Cherokee 5.2 v8 4x4. When the temperature gets above 85F outside, it will usually start straight away. When I run errands I find that after I make about two stops, the vehicle refuses to start. Sometimes it starts, idles roughly for a few seconds, then shuts off. After that it will not start until I let it cool for an hour or two.

If the temp is above 95F outside, it simply refuses to start. Battery is fine, engine turns over, but it wont idle. In temps below 75F, it runs perfectly. Drove it from Oct-April with no problems at all. Being late May, with temps rising, this is occuring daily. It is fine in the cool morning and evening temps, but midday I simply can't rely on it.

I have taken it to my very reliable mechanic, but he seems to find no problems with it. PCM shows all is well. I'm at a loss.