garage door opener problem/question


I've been having some problems with my garage door opener (Overhead Door Python2).
I opened it up and saw that a plastic housing was broken so I got a replacement part and installed it.
However, now the thing is cycling a few inches forward and then reversing back a few inches, instead of operating the full-length of the rail.
It's doing this even while detached from the garage door itself, so it's not some binding or torqueing problem.
Anybody have any suggestions? (Other than using a sledgehammer on it, that is).
Thanks
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