Is this only at one location (Like a seasonal site) or everywhere?
The surge guard only checks incoming power, I think (could be wrong here).
Perhaps your wiring from the socket to the surge guard is loose and the neutral or ground wire is being disconnected by vibration from the AC (enough to cause the fault condition).
In any case it is a problem to be chased down as it can only get worse.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW