Well I've taken the ST out twice now in temperatures in the 50's. No electrical failure. When the temps were above 70 my accessories went out after 20-25 minutes every time. This Sunday temps are forecasted to be in the 70's so I'll take it out then to finish draining the gas tank so I can remove the tank and get at the ground wires for cleaning.
I think Mark is spot on with the thermal expansion opening a circuit up in one of the connectors. Only way to find the intermittent failure one is to test each connector while running after the failure. I figured out a way to do this with a voltmeter and Hitachi connectors, but I decided to replace the Harness, accessory socket, and Garmin. Testing each connector will be a super pain.
The shop damaged my accessory socket and soldiered it back together which slightly melted the plastic. I've been needing to replace this since I've had to fix it already. My Zumo 550 is 8 1/2 years old and has been having a few issues, so I've been looking for an excuse to get a new one and it was on sale. So really I'm only risking the Harness as unnecessary.
Harness, $37 shipped and taxes. Powerlet accessory socket, $34 shipped and taxes ($88 for an OEM is too much and the Powerlet is better made and sealed). Hitachi connectors $14 shipped. Garmin 660LM with ram mount, cradle, electrical harness, and lifetime maps; $409 shipped and taxes ( I paid $600 for the 550 without the ram mount in 2007).
I'll see if the replacements fixes the issue.