Let me start for thanking everyone for any advice.
I have a Pentair Master Temp 400. All our pool equipment was installed in 2008 by previous owners. Everything is pentair, control board, variable speed pump, chlorinator. The only thing I've had to replace is the chlorinator so far, but I'm getting there with the heater. What happens when I fire up the heater, after about ten seconds the pipes start to cavitate , the heater turns off and it sounds like there is a pile of rocks in the pipes. From what I've read, it's a problem (or results from past problems) with water chemistry and things gunked up in the unit. This would make sense since two summers ago I had to fire two separate pool guys for not showing up and/or letting chemistry get out of balance. Ever since that summer I've had problems with the heater.
If I keep my PH levels on the very low side of the ideal range, I've had limited success with the heater...although it shuts on and off trying to get up to set temperature level and it still cavities, but sometimes it can get past that point and stay on. Our water is pretty "hard" where we live (Southern California) and I've noticed calcium flakes in our spa from time to time. Usually when the PH gets on the high side of the spectrum.
I'm pretty handy with trouble shooting and repairs, but don't know exactly where to start and in which order to check things, etc. Again, any advice would be great.