1990 Volvo 740 won't start when warm

This problem is intermittent, never happens when the car is “cold” - it starts right up every morning. When there is a problem this is what happens: I turn the key, you get all the normal lights and sounds: it’ll crank but it won’t “catch” the combustion and resulting roar and running of the engine doesn’t occur. If I give it gas, that doesn’t make a difference. But sometimes, what has worked, is when I turn the key and if it doesn’t immediately catch, I let it go back, then turn the key again, maybe do this 4 or 5 times and each time it begins to catch a little more then finally the engine turns over (I don’t know if that is the right term) and everything is fine. If I try this trick (just turning key briefly, not holding it there) and it doesn’t start to work - if it doesn’t sound like it might almost make it on the first one or two tries - then it never reaches the point where it does “turn over.”

By the way, I think by now you will understand that I am not a car person, but I do appreciate basic scientific principles, so if anyone can provide insight as to what might be happening, I would be quite thankful.

Also, my mechanic is a Volvo master mechanic.

hard to give advice to a Volvo Master Mechanic,espically when he has car in hand,has the mechanic sucessfully determined if this is a fuel or spark situation?

I wonder if the fuel pressure is being maintained while the car sits,or on the other end are we dealing with fuel fouled spark plugs?

yes, he changed all the spark plugs