Just got home from work and it's dark out...but I do see that the frame ground is just sort of hanging there not attached to anything. Could this be responsible?
This... start checking connectionsHard to say for sure that it's the ground wire but it certainly sounds like a wiring issue, most likely a poor connection or solder joint. While I can't say for sure that it's not the ground wire it seems unlikely based on your description of it's current state, as that would most likely cause it to be consistently off.
Intermittent wiring issues often present themselves with fluctuations in temperature due to the connection points being of different density or different material altogether expanding and contracting at different rates. Unfortunately these are the biggest pain to troubleshoot if you cannot consistently recreate the failure. Sorry I don't have any great advice other than to get in their and start gently, slowly and methodically moving wires around until you can re-create the issue.