Thanks for the suggestions. I checked and reset all the connectors, but the problem continued. I think the 'big blue' cap was replaced just before I bought it - I'll check with the seller - it looks new.
I decided to to a self-test. While no errors were found, I think the problem may have been related to either the self-test switch or aux coin switch. After flipping both of them several times for tests, the problems *appears* to have gone away. It hasn't happened in several hours, which is a record since I moved it inside.
Checking voltages is beyond my ability to do safely, so I'll have someone more experienced do that if the problem recurs.
Thanks again all!!