That voltage drop sounds like a bad connection somewhere, which only makes itself a nuisance as things start to warm up. As mentioned by our friend above, try re-making (and cleaning) the HT cable and tube connections. If that doesn't help, look for a poor connection elsewhere, possibly a loose nut, or screw, on a heavy duty connection someplace.
Meanwhile, I'm still pondering this one:-
"relay error with beef sound and code " is flashing.
Did you mean a chattering relay? Or a
beeping alarm sound (the latter, I guess).
