Check ALL your grounds. The main ground on these cars is on the driver's side, its a metal strap that goes from the driver's side motor mount to the frame and VERY prone to rust/corrosion. I replaced mine with a 4 gauge battery cable on BOTH sides so both motor mounts have a grounding cable now. There are other grounds like the points behind the headlights but the motor mount ground is the main one.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.