My 4000 used to do that as well.
I never did figure out the "too fast" BS.
The reason at least for mine doing the quick off course thing was bad solder connections from the four power transistors mounted on a heat sink in the middle of the AP display circuit board.
It's not a particularly hard fix if you know how to solder.
On mine, EIGHT of the 12 connections were iffy.
It hasn't messed up since, unless we get hit with very close military radar
Additionally, my HAM radio
will also cause the same symptom when I transmit on certain bands, but at least I can be ready for that.
Mine keeps the heading memory ok so I've never looked hard to find any backup cell, but yours may have a small ni-cad cell inside which is going bad.