Recently, the thing started downshifting from drive into 2nd gear on it's own - from 30-75MPH or so - which is fun to watch the RMP in 2nd gear at 70MPH. It seems to do it only after the tranny gets nice and warm. There is plenty of good fluid in it.

Is there a component that I could look at or is this likely a internal problem that will require it to be pulled?

Thanks.