I have a 2001 Discovery II and when driving at 50mph (or 40 or 30) but a constant speed, my transmission just shifts to 3 and sometimes 2 and back with no incline or gas fluctuation. Just does it while driving. Does anyone have any reason why?

 

You might need a transmission service that is pretty it would be unusual.

 

you probably have some trans error codes...did you check?
the plug that has a screw retainer into trans can come loose...
last trans fluid / filter change?
bad XYZ switch?