Electrical Problem

[bobpm45]

Hi all,

When I turn on the ignition but do not start the car , all electrics work perfectly. As soon as the car starts , the battery light comes on ,the radio goes off , the indicators stop working ,the rear stoplamps stop working ,the speedo stop working , the electric windows and sunroof stops working !! I have fitted a spare alternator ,and its just the same . I have checked the voltage accross the battery while the engine is running and its showing 13.7 volts. If I then switch off and then back on without actually starting the engine again , all works ok again ! Has anyone experienced this before , or have any idea what it could be .