Red Triangle - P0a80, P3018

Three times now my 06 Prius has given my the Red Triangle and the HV power has gone offline.
The first time a simple power cycle fixed it but these last two times it hasn't. On the second time I drove with it it seemed fine and then the car died and wouldn't go. I turned it off for about 5 minutes and turn it on and it was able to drive home. I read the code and cleared it and didn't come back for about 15 minutes but the hybrid still wasn't working during that time. Ot was, however showing that it was fully charged and charging and in the green (which is rare for this car).
Now what?
Access the HV Batts and test each cell or is there something else that I should be looking at before I do that major thing? Could the secondary coolant pump be causing this?