HP Laserjet 4550 N

My HP Laserjet 4550N is refusing to print, and prints reams and reams of paper before every job. Every once and a while I get "49.2FD3 Service Error".
Piece o' crap! Incredibly inconsistent behavior. Mac OS X Panther 10.34, Windows XP, and even directly from the printer menu: all exhibit the same symptoms.

Update, per the knowledgeable folks at FixYourOwnPrinter, the problem is with the Transfer Belt Unit. A work-around is to open the back cover right after a blank page comes out, then close it again. I don't know why this works, but it does.

Second update: I replaced the entire transfer kit (which includes a new roller), and vacuumed/wiped everything I could, and this has apparently fixed me. YRMBD.