I have a 91 GT and lately instead of the fuel pump making its normal noise the noise goes up and down at idle and when your driving it. also when the fuel pump noise goes up and down the volt guage does also. lately i have noticed power loss and sometimes while driving the car it will move on its own after you let off the gas almost like i turned on the cruise control, and in the morning when i turn on the key the fuel pump sounds like its working and priming the system but the car wont start i have to turn the key on and off 3 or 4 times just to partially prime the lines sometimes three or four isnt enough.when it does try to start it turns over really slow and then it picks up and runs. just wondering if anyone else has had a problem like this. THANKS

 

Not positive, but you may have battery or alternator trouble. The fuel pump relay might be toast too. Thats cheap to replace might wanna start there. Mine was under the drivers seat on the 86.

 

The pulsating voltmeter makes me think the voltage regulator is out. This will also give you a weak battery since it isn't being charged properly.