As soon as it I got it up on the lift, it was apparent that the ground wire spade connector, (somewhat oily) was loosely connected to the spade connection on the solenoid - it was a loose slip fit (how did that happen?). It could be the source of intermittent contact. So I cleaned everything up, tightened up the female connector on the wire and slid it back on snugly. I wasn't able to get it out on the freeway to check it out today, but tomorrow I head up to Sebastopol for the day, so we shall see. My friend with the shop is convinced that this is the cause - he said that as the solenoid heats up it causes somewhat more resistance, and that maybe why it doesn't show up right away. Enough resistance and the poor, oily contact eventually create a situation where there is no ground on the solenoid and so it releases, if that makes sense.

A lift certainly is a beautiful thing.