Probably a bad coil boot, under the load of the engine/trans in drive, it demands more from the coil, and if there is a break/tear in the boot, it will pass to ground the way with the least resistance. Either that or you've got a bad coil, and it can't handle the load. Take out the #8 and #7 coils and swap them, and check it again. If the problem moves to the #7 cylinder, you know its either the boot or the coil.