Start with the easy stuff first.Are all pumps operating properly, sound good? Any water in the pan? Inspect all 3 pumps to confirm the pump seals are not bad (circ, then p1, last p2). If no leaks or corrosion are present, I would move on to Amp draw on the heater to confirm it is working.
Instant tripping is caused by a bad component, and is easily found. Random can be harder, even a bad breaker (only need a 50a for that spa by the way) That year and model spa can be prone to leaks if the water chemistry has not been maintained (plumbing leaks).
Also check the air blower, mice can get in them (black foam in the spa), If an air control is mounted above the blower check the HTC jets to confirm the inserts are not loose. If so, This causes leaking, and they should be replaced.