Check that the reset you are doing is a hard reset rather than whats called a soft one. Manual should explain - I have a Navman and a soft one is pressing the pin button but a hard one to clear major issues involves moving the battery isolator switch to its off position and then back on again. This works in a same manner for a PDA I own as well.
If that doesn't work then contact TOMTOM if still under warranty as although most electronics are very reliable these days the odd fault still creeps in!
Good Luck
Mark R, May 2008