Tachometer problem

[edo@mails.imed.com]

     The tachometer in my 1975 MGB only works part time. Most of the time 
     when I start the car up in the morning, the tach works fine and works 
     as I am driving to work. However later in the day, such as at lunch 
     time or after working hours the tach doesn't work.
     I have not been able to trace the wires from the unit to whereever 
     they originate.If any one knows where the wires from the tachometer 
     unit connect too I would appreciate the information.
     
     This is the first time I have tried to communicate with this this 
     service and am anxious to know if I was successful.
     
     Thanks in advance for whatever info I may get.
     
     Ed