What firmware version are you running? It should have automatically been updated OTA earlier this year to 1.00.15 but that assumes you put the box into standby overnight (updates including channel changes occur at 3am). Given it happens when you change channels I suggest that first thing to try is a full channel rescan:-
Menu -> Setup -> Freesat Tune
I assume that you have not got any non-Freesat channels tuned in which are the ones causing the reboot?
If that does not help then you could try a Factory Reset:-
Menu -> Setup -> Factory Reset
This won’t delete your existing recordings but you will lose your timers and need to set them up again.