I agree with the consensus, of the other posters on board, that the most likely cause, is the idle mixture or its air flow. The FPR on the fuel rail will have little to do with it unless it is very hard starting or no power under certain loads.

You said they cleaned the throttle body plate and bore. I wonder as well about the throttle switch adjustment. You see it tells the ECU computer that you want the engine to idle.

The program in there looks at the RPM signal coming from the hall sensor in the distributor and tells the IAC to maintain 750 RPM. I see no problem coming from it as the engine fires right back up.

If there is an additional load of say the A/C, that is taken care of, all in one fell swoop. You did not say if this occurs more often or not with the A/C on or off. It's just a tidbit factor that might twist a diagnosis more towards the IAC not working within its "idling range of adjustment."

You cannot have the throttle plate to far open first of all.
It needs to be on the throttle stop and the plate sealing off the bore but not binding due to the return spring pressure.
The switch must be clicked when the plate is at rest closed. The switch must click again as soon as the plate wiggles to open but only move the least little bit.
When that plate open air is allowed around it whole outside diameter, of which is actually a lot area that supersedes the IAC flow very quickly.

Upon all startups, "A" properly adjusted throttle plate, "A" good working IAC will race the engine up past its normal idling range and then, promptly drop to idle.

It should do this like clock work! No matter the conditions! Engine hot or cold!
This is and should be a constant "mental note" for the owner "everyday" and when mechanics are there to check upon these of type complaints. Did the mechanics ask about its everyday startups behavior?

Another tweaked in the mix is the AMM or I should say, "The Variable Makers" behind the AMM or any and all other vacuum leaks.
Both vacuum hoses and that "accordion hose" can let air in randomly.

If there is a loose or distorted clamping that can allow an air gap on the ends. Even having the tiniest split in one of those volutes which can pop open and back closed might explain things. As, Who knows when, how much or for how long they last?

All of this happens very slowly over time and goes basically unnoticed. That is, until the amount of air disrupts the mixture beyond the RANGE of instantaneous response of the IAC.

That air flow is really unmeasured.
The ECU has no clue about the "excess" except from the O2 sensor. So it adjusting away, going down the road, until you stop and tell the ECU want to idle now. Hopefully it gets told?

The 02 is still nice and hot. Exhaust flow drops, the 02 throws down a sweep of different voltages so now the ECU can be and is getting lost! UNLESS, the front in adjustments of the air and incoming information is dead on (normal operation) or it's "Catch it if you can time!"
Shudder and more shuddering begins with clacking of spark timing adjustments. All because, "wholly smokes", The RPM signal is totally unreadable into any program.

Bingo it, ECU Defaults into shut down mode to prevent engine damage!

Reset and restart, all is well!

Hopes this helps you! This helped me! I haven't posted in a while. The board has slowed down.
Good luck.
Phil