One other thing to check is whether you have a good ground circuit. Two things, one is paint between the starter housing where it bolts on and the bell housing. Before bolting up the starter, both surfaces should be free from paint. Second, make sure you have a cable from the bell housing to the chassis to ensure that the engine is in touch with ground. Relying on the motor mount bolts is not always sufficient. While the real problem is as described above, having a marginal ground compounds the problem.