I can only make a guess at that, but I would say it could be a bad ground connection, or a problem with the computer. Try disconnecting the battery for about 10 minutes and then reconnecting it, it may act a little weird after you reconnect until it relearns the parameters, but it won't hurt anything and it may take care of the problem. Again, this is just an educated guess, but other than trying to track down electrical problems with a digital meter and oscilloscope, this may be a quick fix. Also check the ground connection at the chassis and block, you may find something simple.