I'm not familiar with the Artic spas innards... but it sounds like a thermistor problem. Your thermistor could be confusing your temperature settings with the lesser temps, which sometimes happens as thermistors get older. What I mean is that your thermistor would be responsible for shutting off your heater once it reached your desired temperature, say 102... However, when it reached 98, it would "think" that, that is 102. Make sense? Kind of?
If it's under warranty, it should be taken care of... If not, I can't imagine that particular part being a bank breaker. I'd consult Artic first before you do anything. Again, this is just a suggestion.