A manifold is a lousy place to terminate the ground on the engine block. Just for grins, get a jumper cable next time and run from where you have the ground attached now to a substantial place on the block or something bolted directly to the block with no gaskets between it and the block (alternator/generator bracket). I had this problem with a rice burner I owned where the ground for the engine ran to the intake manifold. There was a small jumper wire (16 gu) between the firewall and the valve cover that smoked and fried one day and starter acted up after that just like you described for several weeks. Moved the ground to a boss right above the motor mount and never had another problem.