1988 with Intermittent Problem

[renovate7]

I'm on vacation and helping a friend with an intermittent problem. This is a 1988 Marathon with the resistor speed control, serial number 5089XX. On occasion, after driving for a while, with many starts and stops, it will not move. No forward or reverse. If you let it sit for a bit it will work fine again. The other day when starting out for the first time of the day it wouldn't move at all. After numerous key on/key off and moving it from forward to reverse it started working just fine. Took it on a 30 minute cruise around the neighborhood, with many starts and stops, with no more problems. While I can't hear an audible click from the solenoid standing next to it with the seat up it must be working as the cart will move fine 99% of the time. This is a very intermittent problem as I have been out 3 times for cruises and it hasn't happened to me, other than the no move when first driven for the day. I've checked for loose wires at the on/off switch and the small wire to the solenoid. I have no test lights/volt meters etc with me and about all I can do is change out the solenoid. (I hate being a parts changer rather than diagnosing the problem) Any help appreciated.