If you have a problem with a slow cranking mustang, and it gets worse if the engine is already hot, then check your main groud cable. It comes off of the negative battery cable that runs along the front of the motor at the top of the oil pan. It's a metal band that is bolted to the block, right above the oil pressure sending unit, and in front of the oil filter. If it works it's way loose, it can cause all sorts of electrical issues.