mine went out while still under warranty... seems to be common just like the window motors. Replaced that too (passenger side)...only this time I was like a month out of warranty. lol.
These motors tend to get alot of carbon build-up which slows and stops the motor...some ppl have taken it apart, cleaned it, and put it back together....some work for a while, but they eventually die too. Just buy a new one.
Strange that a Z33 seat motor would do that. I suggest if you're handy to open her up like old school suggested. Could be a loose connection or a bad brush in the motor but if it's under warranty then don't touch it.