Charles
Check the grounds from the body to the motor (left head) and from the body to the transmission.  Also check the battery ground under the passenger seat and the dash ground.  I have also heard that the mixture of anti-freeze and water can affect the sensor (make sure that the sensor in the pressure tank is clean and that the coolant mixture is correct).  You may also want to double check the relay.
That should keep you busy for a minute.
George,
'85 Westfalia