We have a 1995 Hot Spring Highlife that has recently developed a problem. The heater is rapidly switching on and off, and this is making quite a racket at the heater relay. I have checked the voltage at both the panel and at the heater element, and it holds steady at the heating element at around 230v except for very quick drops and back up when it does this switching thing. Turning the thermostat switch up and down also gives rapid on and off control of the heater, and I think I remember there being a delay in turning back on after turning down before this problem developed.

I am guessing either the Interlock Timer Circuit Board (main board) or Thermostat Control Board. These parts are not cheap though, and I hate the thought of replacing perfectly good parts until (or if) I am so lucky as to have the problem go away.

Anyone out there with Hot Spring experience that has encountered this problem and has an idea as to what's going on and where the problem might lie?

Thanks!

It could be the flow switch. Have you ever had the heater replaced? If so the flow switch may be jumped and then wouldn't be the problem. Try removing the filters to see if this helps. Low flow from the circulation pump could make the flow switch open & close. If it stays the same try jumping the flow switch. It should be the bottom connector on the side of the control box.
Have you tried replacing the control thermister? It is Part# 38415. This is only about a $20 part. Usually it is this part when you get lots of clicks, especially when slowly turning up and down the dial.
If that doesn't do it I would suspect, like you said, the t-stat board - Part# 71340 about $65, or the relay - Part#34321 about $16.