Sundance Aruba w/SBSG LX-10 circuit board (now toasted) wired for 220 trips breaker at house mains panel (50A), but not the 50A GFCI at the tub disconnect box. Heater seems to check fine at 10.5 ohms across terminals with no apparent short from either terminal to grnd. That stated, breakers
held w/o heater connected. Re-connected heater and seem to have had a
short (serious arcing) between the two 110 main terminals feeding the main
contactor on the board. Pulled the tub feeds off the GFCI and it tests fine w/o
the tub in the mix. Metered wiring from tub disconnect to circuit board for shorts and came up clean. Questions are: Still a bad heater which tests good?
Bad main contactor short that only happens with 220 demand from heater causing excessive current draw, but no grnd fault to trip the GFCI and mains
breaker is just more sensitive to such a condition? Now what?
BTW, the GFCI does trip occasionally from power surges/spikes after a power outage. A reset brings the spa (well, used to) back up with no problems in the past. All has been fine for 7 years. Thanks in advance for any help or advice.

Arcing between the mains? There should be some carbon and damaged or burnt product there that needs to be removed. I would think that maybe one of the main's connections is loose and causing the arcing on heavy draw. Best pull the meter out then inspect all the connection onto the main breaker. Black, dark, brown connections mean poor and overheated connections. Fix it. Maybe install a new main breaker.