I have an 87 mustang 2.3 liter with 5 speed transmission. When it runs and warms up, the idle comes down like usual, but after a few minutes it starts to idle rough and begins to plummet back and forth, if you push the accelerator..it pretty much dies then will catch itself, or will just shut off. I have also noticed that when I turn things on like the heater and headlights, the volt guage drops and you can hear the fuel pump get weaker like it isnt getting suffecient power. I have changed TPS, ECM, battery cables, fuel pressure regulator, plugs, wires, starter relay, rotor cap and button, timing belt, egr valve, pvc valve, alternator, battery.... I dont know what else to try. I have been told it sounds like bad grounds.... but I dont even know where to start..any help would be appreciated...thanks