Weird (to me) Problem with 05 Prius - Contactor?

Hey all. I've been dealing with this issue for a bit now, and I'll try to give you as much diagnostic information as I possibly can.

Symptoms
Occasionally while driving, particularly while coasting downhill or maintaining a cruise speed, a clicking is heard from the back of the vehicle. The vehicle then revs up to ~3800 RPM and stays there with little to no throttle response. There is a significant loss of power and responsiveness. When you shut the car off, it slowly dies down, and restarting the vehicle alleviates the problem. Putting it in park and then back into drive, or any other mode, does not help anything.

History
This vehicle was kept garaged by my ex, who allowed the battery to die completely. When I retrieved the vehicle after some months, I jumped it and drove it. There was no MFD or HUD on the first drive back to my house, but they returned when the vehicle was restarted. The vehicle drove fine the entire time, and for some time after that.

Tests I've done/Relevant info
-It is apparent that HVB block 9 is bad - by the P0A80 and P0319 codes, as well as my scan tool indicating that the voltage is ~1v or better below the rest of the blocks.
-Hybrid battery is not overheating.
-Hybrid battery fan runs on max speed constantly from the time the vehicle starts
-Aux 12v battery is still good
-Took a drive with the cover off of the contactor area of the hybrid battery box. It SOUNDS LIKE the contactor quickly unlatches and re-latches when this 'transmission slip' starts happening. I still get HV from the negative side of the contactor after this happens (I pulled over and tested it to see whether the contactor had completely unlatched).
-HV Battery Current reads as 0.02A after this happens, regardless of inputs. HV Battery Current reads normally until this problem occurs.



Any insight? Thanks in advance