Maybe you should rely on the high tech computer system instead of the 89 cent plastic thermometer.
Only kidding, definately check your Micro Clean filter, if it hasn't been replaced over the last 3 months, replace it.
To check to see if that is it, remove your filter completely, make sure the spa is on standard, check the spa one hour later, see if the temp goes up. If so, replace the Micro filter.
Best of luck.