Ever since I replaced my wiper motor (1990 245) the wipers like to stop mid-cycle (i.e. in random locations on my windshield other than down). What's happening is that the shaft passes through the firewall and threads to a nut that holds the linkage to the wipers, and that nut keeps coming loose. It's a pain to tighten, and I wonder if I'm just not getting it tightened properly. The routine is that I get under the dash, reset the linkage so that it's "zeroed out" and aligned, and tighten the nut. The wipers work great for a while, but over time the nut must loosen and the wipers end up stopping in the straight up position on the windshield. What the heck am I doing wrong?