I realize that these synchronizers are a recurring theme, but I have searched the forum and can't seem to find an answer, or the exact same problem. I have an 04 Mustang with the 3.9. About 2 weeks ago the synchronizer started raising hell. I didn't get any chirping ahead of time or anything, it just started screaming. Anyway I get a new one from Napa ($199-makes me wish I had the 3.8) which came with the new sensor included. I had talked to a mechanic prior to trying the install and he told me that I could mark the locations of the old unit and line everything back up exactly to avoid my concerns about not finding exact TDC. I did that and the car seemed to run fine, but after a few hours I got a "service enging soon" light. Went to Advance Auto and sure enough the code was for the synchronizer. Assuming I got something off a bit, I researched TDC and set it, then used the tool that came with my new part and installed it correctly. Car ran great again, for about two days. It started to jerk around 35-50 mph and then finally threw a flashing trouble light. I took it to a trusted mechanic and he hooked up his reader and we got a no. 1 cylinder misfire. We pulled the no. 1 plug just to see if I bumped it or something during reinstall and found nothing. We also used a spark tester and the no. 1 was firing it's butt off. As of now, the car is still at the shop and he can't seem to diagnose the cause of the jerking. The service engine light has not come back on, and he rechecked the installation of the synchronizer and it is good. One thing I forgot to mention; the new sensor that came with the synchronizer would not plug into my wiring harness. It looks almost identical, both are two prong, but the male connectors on the sensor are just slightly larger than the female connectors on the harness. Instead of altering a bunch of stuff I just used the old sensor since it was the bearing of the synchronizer that was bad. Any ideas would be tremendously appreciated. Sorry for the length of the post. Thanks.