I am not familiar with you model, but I recall doing a search for sunroof problems and the problem you describe might be related to the sunroof motor clutch.
Try doing a search for motor clutch and see if it might apply to your problem
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles