2005 Toyota Prius - Red Triangle - Have Tried So Many Things to Fix It

Hello, I am reaching out here because my husband and I have a 2005
Toyota Prius that we love and want to keep running. BUT, it has given us so
many problems that we are almost down to the last straw.

When we turn the car on, it shows us the check engine light, yellow circle
with the exclamation point in it, and the car skidding light but does not start
the engine. After using a code reader to clear the codes, the car turns on but
about 10 minutes down the road, it shows us a red triangle. The gas mileage
is down and the car runs very choppily (the engine kicking on and off almost
violently - definitely shaking the car, especially when waiting at traffic lights).

We have used a toyota specific code reader to get the codes B1421, C1241,
and B1271.

We have tested each cell of the hybrid battery (all good) and cleaned the
corrosion from the metal parts.

A couple of months ago, we took the car to the local Toyota dealership and
they told us it was a problem with the inverter/converter. We bought a used
part, replaced it, and it didn't help.

We would really appreciate any insight/help you have for this problem. We
love this car and don't want to get rid of it, but we need it to work correctly :(

Thank you!