I think I can help you with this one as I've had the same problem myself
and it's driven me nuts but it's now sorted with help from Andrej.

I bet you're using an external GPS on Comm 4 and have Use backup GPS
checked in HardwareSettings?

You must UNCHECK Use backup GPS and while you're there uncheck Use GPS
forwarding which is an LNav only thing.

There's a conflict in there somewhere that causes the backup GPS to kick in
every time it sees the slightest degradation of Primary GPS signal, even if
it's only momentary, so it then cannot make it's mind up which to use and
you end up with this continuous and very annoying bouncing backwards and
forwards between the two. If your using a good primary GPS input you don't
need the backup facility anyway.

I've now had several weeks of trouble free operation after suffering this
problem on almost every flight.

Garmin 2+/ LX Nav V&/Oudie 2 at 4800baud.