**UPDATE**
After talking with Pentair tech support (these people are hit and miss, had a not-so-helpful conversation the first time, the second time we actually diagnosed the problem), I replaced the circuit board which was a fraction of what the pool company was charging ($600 + installation, but since I was a return customer he said he'd install it for free), and voila! Pool heater works like a charm! Hopefully others with this heater and similar problems can take note of my adventures in heater repair.
