1994 F150. The amp gauge snaps to the left towards the - (negative but within the normal range printed on the gauge) side and then back to normal, towards the positive side of the dash meter. This is a real quick snap. You can hear it effect the idle and this also causes headlights to dim, heater blower to slow down etc... This only lasts a quick second or two and everything returns to normal. Truck wont stall or shut off YET.

I'm thinking and looking at bad ground, bad coil or perhaps bad ignition switch. Truck has over 100K. Any other suggestions ?