66valient just dies

Check out how your coil is mounted if it turns out to be okay, especially if it's an aftermarket. Even though bolted into the stock position on the intake manifold, it's horizontal -- move it upright to the firewall and watch the problem disappear. According to the MSD guy, laying on its side leaves too much room inside and the cooling oil doesn't reach the area it needs to. Had the same problem on my '83 D150 and repositioning the coil fixed it right up.