We have a used cal spa given to us by a friend. Have had it 4 months, working perfectly. Last few weeks it is alerting OH and the temp drops. Can kick back on by simply hitting jet button on topside controls. It However now is kicking back off after a period of time. I removed the filter cartridge to see if this was cause with no luck. This evening, showing OH, pushed jet button, temp displays 108, push temp button to lower, says 100. Any advice would be appreciated, this tub is really in nice condition.