Offshoreonly.com - ignition --timing

Usually the sensor in the disributor either works or it doesnt at all. Ive never run into a wacky one, they just quit. I didnt want to mention the distibutor gear so as not to scare you, there are other cam shaft issues that could do this too, if the cam was walking forward and back. Pray its not that. if you check and verify all the easy things, then the dist gear and cam shaft end play are next.

The modules are expensive from merc, try and borrow a know good one from somebody to trouble shoot before you buy one, the old ones seem to last forever. Im still running on the old t-4 metal boxes with pigtails.