My passenger side went out last week. This is after replacing the regulator on the driver's side, and fixing the passenger's side previously.
New parts that are PROPERLY installed, will buy you some time, but this problem is never going away.
My belief is that the window goes out of the track not because the grease gets hard, but because the front and rear rubber/felt guides bind up.
There are black rubber/felt guides in the front and rear of the door. When the window binds on them, they either shift, or fall out of the metal rails that hold them in. That is what eventually dislodges the window.
I just opened the door, and put the rubber/felt guides in place, and reinstalled the window. No guarantee this will last forever, but it should do ok, until it happens again, which it will. Also, makes sure the bolts that hold the regulator onto the door are tight. If they are not, they could cause the binding on the guides and cause the problem. __________________ 
1998 (July) 318ti, 5-spd
OEM Armrest Blaupunkt Heidelberg CD50 CD Player/Compact Drive MP3 Player Diamond Audio 5.25" rear speakers Navman ICN 530 GPS BMWALARM.COM (with comfort settings) after market alarm system Magnecor 8.5mm wires M-Z3 Shifter/Momo Knob Burlwood Dashboard Stromung Exhaust X-brace Racing Dynamics Front Strut Bar Carbonio C.A.I. 17" Rial Rims Vader Seats/Heated/Lumbar Support M-tech Steering Wheel/Front Sway Bar/Front & Rear Bumper Depos/w 6500K Angel Eyes/6000K HIDs Clear Corners M3 Mirrors UUC Light Weight Flywheel/M5 Clutch/M3 Clutch Slave E28 3.46 LSD/Mcoupe Cover/E30 Flanges & Halfshafts UUC S.S. Brake/Clutch Lines Hartge Roof Spoiler BavAuto Springs Bilstein Sport Struts/Shocks E46 M3 Rear Shock Mounts SPC Front Camber Kit Reiger Hatch Spoiler BavAuto Rear Camber Kit Dinan Stage II Software Turner Rear Sway Bar Reinforcements BMW E46 Auto dimming mirror with Clown Nose alarm Engine Compartment Light Heated Wiper Fluid Retrofit OEM Fire Extinguisher Da'lan Trailer Hitch Rear Sun Shade OEM Fog Light Retrofit H & R MZ3 Rear Sway Bar/ UUC Adjustable End Links Cruise Control Retrofit On Board Computer Retrofit M3 twistie style side skirts Carbon Fiber Hood |