Most garage door openers have a feature that when they encounter too much resistance, they will reverse. This is to prevent the door from trapping/crushing something that didn't get out of the way (a car, person, or your dog). It sounds like the door isn't jamming completely, but it is just "rough" in one spot of the travel, which kicks the the opener into reverse.
Do you have the option of shutting the door manually? In my garage, there is a latch that lets me disconnect the door from the opener, which allows me to slide it up and down by hand. Try opening/shutting the door hand and see if it feels "tight" at the point where the opener would reverse.
If you can identify a spot where this happens, you have something off in your door/track.
If not, something is wrong with the garage door opener.
If it is the opener, some things to look at:
Some openers require periodic
lubrication of the screw drive
mechanism (not the door track). Find
the manual for it and see if there
are any reccomendations for greasing
it. There may also be crud in it,
preventing a smooth travel.
The electronics inside the opener
that detect a jam may be
malfunctioning. If so, good luck
finding replacement parts.
One other things to note: Did it suddenly get cold out? Thermal expansion properties of the door and track can trigger problems.