2005 Toyota Prius Driving everyone crazy!

Ok, so a little over a year ago my daughter got a 2005 Toyota Prius and it worked great for several months. Then one afternoon while driving it she got the red triangle of death and we put it on a code reader and the code was for the inverter pump so I picked up a new one and we changed it out, no problem. Three months later she gets the triangle again with the same code. We checked the pump and it didn't seam to be working so we changed it out again but this time a few days later she gets the triangle again but the pump is working fine so we check the 12v battery and find it is reading about 9.6 so she buys a new one and we put it in.
A few days later triangle comes back on. Pump is find battery is fine so we start checking fuses and pull the fuse for the pump and other then a bit of corrosion its fine. So we put it back in and the light goes off and stays off for a few months before she gets it again. Still showing the code for the Pump. We check the pump get a new one and replace it but the light is still on.
So we check the battery the fans and the hybrid battery all fine. Put the car back together and the light is off. She drives it for a little over 2 months before the triangle appears again. This time we take it to a mechanic and he says the problem is the radiator fans.
We buy new one he replaced it and drives the car around say the codes are gone and the car in fine. She drives it less the 24hrs and again gets the triangle. Takes it back to the mechanic and he is getting the same code as always but can't find a thing wrong with the car.
He calls another mechanic that specializes in the prius and they just tell him "no problem, bring it in and we will replace the pump for $300.
Since we already know the pump is working this isn't helpful.
If anyone has any ideals on what is wrong with this car please help.