It is becoming quite a bugger. I'm going to try jumping the SSR to see what that does, I've ordered a new SSR just in case (as you said, I should have another on hand anyway), and also a new thermocouple. May be a long shot, but if part of this one was submerged it may be causing some problem. An EMI filter would be ideal, but they're starting at $180 from Automation Direct, soooo I'll be trying all of the $5-10 solutions out first.
As for the cables, they come out of different sides of the box, and don't really run together. Also, when I was testing the mash vs. a regular pot (with one working and one not), they were close together and the probe didn't move at all.
It's very strange, and my experience here is limited, but with all the tests I've done I have to believe there's some kind of interference affecting the probe when it's in contact with the elements being powered by the control panel. It should also be noted that BOTH the heatstick and the cooler I've installed the element in do not trip GFCI outlets when plugged straight into them, so I'm confident those are wired/grounded correctly.
Plus side is, although I had to sit there for an hour monitoring my temps (which is why I made this thing in the first place ), had a decent brew day and should have an awesome simcoe/amarillo/galaxy IPA coming up in a few weeks.