Both front seats in my 1997 960 wagon are jammed. I traced the problem to the actual mechanism on the seat rail that moves the seat (gearbox). The motor works, and the cable drive appears to be working. When I attempt to move either seat, one side will move enough to get the seat cockeyed, but the other side holds firm.

I removed the drivers seat and disassembled as much of the mechanism as I could and it worked when I put it back together. But after a couple of days, it was jammed again.

Yes, I'm certain that there isn't anything blocking the seat tracks - but what's odd is that both seats failed at about the same time.

Is there a way to make a DIY repair, or will I need new seats?

You are close to the answer. What is happening, the cable is squared on the ends, one end is rounded off and is not being driven by the motor or the other end is not driving the gear. Another thing is there is too much room within the cable sheath (poor engineering), allows the inside to travel and work it's way out of the gear. You will have to take the seat out, some have just shortened the sheath by taking off one of the metal ends of the sheath, I hear a lighter works great, heat up the metal and melts the plastic, snip off 1/4" inch or so of plastic sheath, rebend the crimps on the metal tip and place back on the cable.

What I did was a little simpler, disconnect at the motor, turn the inner cable until it slips back into the drive gear. Measure how much is still hanging out the motor end of the cable, measure how much room is in the motor shaft place some stiff wire to fill up that space in the motor shaft. This will prevent the cable from working it's way back into the motor shaft and the other end moving away from the drive gear. Did that about 10 years ago, still works great.

DanR '94 964 332,000 miles (98,000 on the new engine)

You guys seem to be having a similar problem that I am having with my 2003 S80. The front passenger seat seams to swivel when you push on the button to move it back. The the seat stops. Do you know if there is any other reasons in a 2003 S80 or should i follow your suggestion? Approx. how long did the process take you so I can guestimate how much time I will need?
Appreciate your advice.
Thank you
Phil

Took a couple of hours, if your seat is just moving on one track and starts to twist, then yes do it.
After moving the seat up off the floor at it's highest and allow for getting a wrench on the each of the four bolts, disconnect the negative battery cable to dis arm any airbags. You will need to have your radio code handy when you are done.

DanR