To 300Turbo:
Unless you are a purist, key with the sunroof is to keep the tracks clean, the gearcase friction properly adjusted, and the motor end of the cable lubed with black lithium. I used beeswax on the roof tracks and on the headliner. Smoooooth....
To gcinque:
As indicated in my post, carefully disassembling the sunroof will make things clearer IF the problem is in that vicinity. Please use a Rubbermaid wash tub or similar to receive all parts lest they escape. Once the C-pin is removed from the front of the slide tube, releasing the forward end of the cable, nudge the switch to determine if the motor, gearbox, lower end of the cable are OK.
I also tore apart the motor/gearbox end of the system. The coged drive gear in there interfaces with the sprirals on the end of the cable, causing it to move. You may or may not have some broken cogs (gear teeth). Advise if you want inspection points to look for, R&R tips, photos, etc.
Good luck.