only during hot weather above85 degrees,after longer drives(15mi.or more) or numerous short re-starts,car cranks but won’t turn over or start but as soon as in gear will stumble and die.After 15-20 minutes starts and runs fine.It’s only happened 8or9 times over the past three summers,over about 30,000miles.Fuel pump pressure is ok. getting spark,so i replaced pcm and fuel pump relays didn’t that didn’t fix the problem.Car is 99taurus 3.0L 97,000miles.
Wait a minute, if it cranks it turns over; if it only claks, it doesn’t turn over. Which is it?
Have you had the battery checked? Last resort could be the starter going bad.
“Turning over” seems to mean different things to different people. I would say it is a poor term and should not be used.
It cranks = the starter turns the engine
It fires = The spark is setting off the fuel. It may or may not be starting.
In either case the OP appears to be saying it will crank. However he does not say if it cranks normally or maybe slow.
Assuming it is normal, I would suggest checking for spark next.
siay0859 buy yourself a spark plug and the next time it happens, remove one spark plug wire and put the news plug on it. Hold the spark plug so the threads are tightly held against the engine and have someone try to start it while you look for sparks.
Note: If you are not holding it with a well insulated tool, you will likely see stars if not sparks, that’s high voltage (which would tell us the spark is working 
It would also be nice if you could do this in a dark garage or at night as it is easier to see.
Good Luck