I just sprayed some lithium grease on my tracks last night. HUGE improvement. Mine never shook or vibrated, only got VERY slow when closing. I sprayed the grease the entire length of the track and open/closed it about 10 times. Works like a champ now! It sounds like you may have something in the track that is impeding the plastic piece from sliding smoothly. I don't know what it could be since you said you cleaned it......? *shrug* Good luck. When my sunroof was struggling, it bugged the crap out of me.
In fact, the grease worked so well, I may try to use it during any "alone" time I may have.............j/k
