F20 Fault?

I can switch on my F20, enter a post code & the F20 finds the route okay. However another time when I switch it on I don't get the first verbal instruction & then cannot scroll down the whole route, all I get is the route intermittently and a message across the bottom of the device saying "back on track". It seems as if it will only work correctly if I get the first verbal message for the route.
Help please.