spa thermostat problems

I wondered if you've ever run into this problem I'm having with a tub. I think it's haunted. I have an old Pacific Marquis(about 13yr) that occasionally kicks on high pump speed and heater till the water temperature overheats-all by itself. I'm on the 3rd thermostat to try to fix, I replaced the soldered in pushbutton motor speed switch, replaced power supply cap on board, replaced both mechanical timers, cleaned/checked all relays-it still does it. It's such a simple circuit I just can't understand why it happens. Without actually pushing the mechanical switch, the pump should not turn on high speed-at the same time the thermostat allows the water temp to rise to cutout. It's almost like I pushed the pump button and ran the thermostat up myself, but I'm not dreaming. Any thoughts? Norm
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