I have noticed a lot of people asking this same question on various sites. I found the solution to my problem and maybe yours. My problem was I like to test things before I put all the parts back together. When replacing the worm gear you have to take a plastic cap off along with bearings and washers to pull the motor out. I replaced all the bearing along with the worm gear and tested the opener. It would go in one direction for a couple of seconds...stop...and then go in the reverse direction for a few seconds. I tried everything to fix this. However, the only problem was the black plastic cap that fits over the bearings at the end of the motor shaft. This plastic cap (when operating) turns through the RPM sensor. Without it turning in the RPM sensor the motor automatically would stop and reverse after a few seconds. Took only about 1 to 2 seconds to pop the cap on and now the opener works perfectly. There is no mention of this on any troubleshooting or in the directions for installation of the worm gear. Hope this helps someone else.

Carp is correct. Also make sure the RPM sensor is plugged in as well.

Carp,
This sound like it may be my problem (see Overhead Door posting earlier) I'm not a door expert: is there a diagram or picture showing how to get to the black cap and would this be true for the OHD model 551 chain drive unit?
Thanks!