The spa is above ground. Nothing has changed with the setup of the spa since I bought it new 6 years ago. The only thing that changed was the main pump in the spring when the original died...then 2 months later when the heater died. Both were replaced within a week of failing.
Here's an interesting detail, though...the replacement pump seems to have more suction than the old one. It was strong enough to create a small whirlpool effect over the filter and suck in air for a second, which caused it to "pulse"...it'd do this pretty regularly while the pump would run. I fixed the surging by placing a plastic object near where the whirlpool would start that interrupted the whirlpool...so no more pulsing. Either way, still strange that the new pump seems to have more suction...not sure if that could cause the temp to run up.
Also, a new development this week. I found the troubleshooting manual for the Jacuzzi J-235 online and decided to test the temp sensor and hi-limit sensor. The hot tub had been unpowered for the past week while I was testing things...but when I plugged it back in, I got a SN2 fault...which is the regular temp sensor. The multimeter readings were OK, but very well could be out of tolerance at the upper end of the range...so who knows.
I decided to swap the pins of the high limit sensor and the temp sensor on the board to aid in troubleshooting...and when powered back up, the SN2 fault was gone. So the main temp sensor is definitely bad. Replacement on the way.
However, while I still have the hi-limit sensor acting as the regular temp sensor, I decided to let it run a few days to see if the temps would creep up. The first couple of days they were great...but this morning, when I checked the temps were 3 degrees higher than the set temp on the panel. Verified with an external thermometer. The heater was "off", as it should be since the temps were already above the set temp.
So, my new question is this...would it be possible to have a bad replacement pump that's somehow increasing the water temp? I know a lot about electronics, but not much about fluid thermal dynamics. How can the heat from a pump transfer into the water?