1995 Plymouth Voyager Stalling in Hot Weather

In hot weather after some highway driving followed by stop & go traffic, my 3.3L 1995 Grand Voyager stalls with a little bit of warning, and I'm able to reach the shoulder by excessive gas pedal pumping -- otherwise the stall happens right away. It will not restart immediately - restart takes at least a 15 minute wait - and the longer I wait to restart, the longer it will run before stalling again. After sitting a couple of hours it will run fine. And this past winter there was no stalling at all, but on the first hot day this spring this condition from late last summer returned. Two different, reliable family mechanics can find no codes or any causes.