Becker Navigation Problem

Question

Hi. Have just installed a Becker 7949. The unit seems unable to calibrate correctly. It can find a 3D fix once I reset the unit, and can tell me where I am on start-up, however it continues to say that I am on the same road throughout a complete journey. All gps and gyro funtions appear to be working correctly (and move with my journey) however I have noticed that the 'wheel' value never changes from zero I have also noticed a thin horizontal electrical interference 'pulse' type flash which regularly (every 10 seconds or so) flashes on screen. Is there something that I can do or should I send it back? Any advice welcome thanks.