It's unlikely that there is something wrong with your hot tub. Intermittent tripping of ground fault circuit breakers is most often caused by faulty breakers or wiring. On the other hand if you experience instantaneous tripping of the gfci it is most often caused by a faulty component.
You've had the spa since July 2008 and you're complaining 19 months later that it has only worked 60% of the time? When you find the water lukewarm is the red power light steady or flashing? The spa high limit can "trip", which is reset by powering down the breakers, but that's not the same as the breaker tripping.
If the red light is always steady when you find it lukewarm, and the heater breaker is in the tripped position, there are really only four things that can be the cause: the heater, the heater relay board, the breaker or the conductors (wires). I've fixed several Hot Spring tubs that were experiencing intermittent tripping of the heater circuit breaker (there are two breakers in a Hot Spring sub panel). The first two I started at the spa on both of them, replacing the heater first and then the relay board. Neither time did that fix the problem. One ended up being the breaker, the other ended up being the wiring between the sub panel and the spa. Now when confronted with intermittent tripping I start at the breaker.