1989 Ford Mustang Wont turn on as if battery is dead

It usually turns out to be heat from the engine affecting the starter, especially if it starts OK after sitting for a while, but cranks slow when hot, also check to make sure the engine isnt running too hot, and check base timing by unplugging the spout connector and checking the timing with a timing light.
Engines will crank slow if timing is too far advanced, and it will ping or knock under load! Battery and cables can be a problem too. Check for voltage drop at cables, especially the negative cable.