SatNav losing satellites

[juk]

I have had aGarmin 660 for a few years now, however recently it seems unable to hold on to satellites for more than an hour & usually around half an hour.If it is switched on & off or reset it usually finds the sats again but only hangs on to them for a few minutes.

Any ideas why this might be happening? I have not updated (ie paid for) the maps since I've had it but surely that's not the reason?