Vehicle is used for mail delivery. outside temps in the 90s. two&half hours into delivery the engine stalls periodically. after few cranks it starts. temp gauge & radiator ok. oil pressure gauge is low. dip stick shows sufficient oil. after i let it set for one hour it ran without stalling.

loose/weak grounding cable tied somewhere to the frame/starter/engine

Likely a fuel delivery problem.

Check the filter, as it may be clogged and due for replacement. To test for fuel delivery, open your airbox and spray some starter fluid (purified ether, usually) down into the throttle body while someone else cranks the engine. If it fires up and runs for a few seconds but won't run without spraying the starter fluid, then you know you've got spark and compression. If that's the case, there could be any number of potential issues: throttle position sensor (not very common, but it does happen), fuel injectors clogged (common if you use low-grade fuel), fuel filter/strainer clogged (filters should be replaced every 30k miles or so, on average), fuel pump failure (you should hear a buzzing from the fuel pump when you crank the engine).

If the starter fluid doesn't help, then the problem is no spark. Check each plug/cable for fouling.

Using excessive fuel cleaner can damage injectors and the fuel pump - sounds like your problem is the fuel pump bearings, and you will probably have to have it replaced.

I think that sparkplags get wet. Replace it.

can be any of a million things, vehicle details help!!

This is the same problem I had with my F350 Super Duty. I had taken mine to the shop and 700.00 dollars later still no fix but I found a smart mechanic that told me about the "crank shaft sensor" located in the front of the engine about half way up. Its only 2 bolts but a bit of a ****** to get to. I paid $40.00 for the part and it fixed my problem. Just a bit of a warning~~my trucked started out with the "bucking"and such then graduated to completely dieing on the freeway so be careful.This is actually a Ford re-call on some makes and models. Good Luck! Hope this helps!

It may be the crankshaft possitioning sensor. Hope this helps

Checking for spark is really simple while fuel delivery is more complicated because of the the way it is metered for delivery. On VW's a fuel pump relay is used and a jumper cable connected to relay box. This controls the fuel pump to deliver fuel manually. With a fuel pressure gauge connected to the line from the pump continued withing the system, you assess whether you have correct fuel pressure, maintain it within a specif duration. Electronic fuel injectors do stick or stay open causing very poor or no start condition. Make sure you have fuel delivery since you are positive that spark is present.

have someone check the reading on the engine coolant temp switch,if your switch is telling the computer it is 50 degrees the engine will start it lean,when you really need rich,like an old fashion choke.

My wifes vehicle did that for about a year in hot weather also.
It was giving PCV valve codes, misfire coded, but finally after her insisting I was wrong, I replaced the fuel pump and fuel filter and it has been running fine ever since.
it would run great when it was cool, then when the outside temp rose into the 80s and 90s it would run about 30-40 minutes and just shut down. Let is set, it would run another half hour or so.
Yeah I understand your frustration, its hard to determine also , even with a gauge by the time you get things disconnected and hooked up, it checks out fine.