Looking for some salvation on this as its greatly annoying me

Just recently , my satnav has been giving me grief . For no apparent reason its started to lose its signal and bears no relation to where I actually am ?

It might do this for a couple of days then return to normal , only for a day or so lose signal again . It never shows how many sats are connected even when the nav is working .

The clock doesnt lose time so , not sure whats happening but its desperately upsetting .

The car is a 55 plate CLS .

Any advice to resolving this issue would be most appreciative , thanks in advance .