What year is your na motor? Iv only seen it once or twice but on the 90 model the splines make contact and wear down the points. If you pull the cas out and look down in that little splined shaft there should be like square cut gear teeth. These wear down to points leaving some wiggle room in the shaft and the cam itself so the cas won't read timing right and will jump a lot just like you said. Can you take a pic of the old cas? Take a pic of the back looking straight down the shaft. Just curious if this issue is something to make a sticky about like a common failure.
