from your description of the problem it sounds alot like the system is seeing a problem and generating a PMT fault causing it to reset making you cycle the key.
There could be a problem in the contactor tips bouncing when one or the other pulls in, it could be noise in the fwd/rev circuit that the PMT driver is picking up on, or from any of the other associated circuits like seat switch, accel switch, or even one of the components on the control panel failing the startup sequence.
These driver modules are small square blocks with 4 terminals sticking out of one side and most likely are stacked on top of each other inside the control panel area where the contactors are. You will need a manual schematic to identify which one is the PMTD module. The fwd and rev driver module is also in those square blocks along with the 1A driver.
If those tech's that looked at it weren't older guys with years of experience chances are they've never seen an EV1 panel (in person).
Those went away 25+ yrs ago.
So yeah, they might have a problem knowing how they work and how to check them correctly.
You can find info on the control panel on fsip's website
fsip dot biz and search in the 'Resources' tab and find technical documents and locate the GE Controllers and find the EV1 B C D link.
There is a troubleshooting chart in there but it is not mfg specific, it is only the control panel itself. Like i mentioned earlier, you will need a Yale manual for that truck model.