1990 Ford Mustang gt could hear idle speed rise and fall

1990 Ford MustangIdle speed rise and fall

Car started but could hear idle speed rise and fall. Headlights or radio did not work any more. As soon as put in gear car would stall. That’s about as simple as I can make it so some idea will be appreciated. 🙂 1990 ford mustang gt 5.0 liter EFI. distributor type. 4 speed trans.

Sounds like an electrical issue that is most likely related to some poor connections at the battery.