You know, I had a very similar problem several times, just like this one. What I found was I had a wire rubbed through in the control panel, on the left side. One of the wires going to/coming from R10, R11, R12. The only way I found this, was at night. We were working 24/7 trying to get a Unit ready to deploy. I was trying to take voltage readings on the R10 or R11. It was dark in the control panel, and when I pushed S5 up to close the CB, POP, I saw a spark. If it had not been so dark, would have never found this fault. Or not soon, any way. It could be, that sometimes the wire is not making contact, and vibrations sooner or later move the wire, making contact. Or, it may make contact, like right now, and there has not been enough vibrations to move the wire.
Also be advised that its not uncommon, for the "posts" that the wires are are soldered to on the resisters, to have a bad solder joint. Check them out with a fine eye.