I just experienced the same problem. I took my saw appart. (it's a little complicated but if you pay attention to how it comes appart, you can put it back together) you must first remove the blade assy. Then, use a Phillips screw driver, remove all of the screws that hold the cowling on. be very careful when opening the cowling. You don't want to jerk anything loose. Use a pair of small snapring spreader pliers and remove the snapring from the gear assy. (this is what turns the cutting chain ) After removing the snapring, gently pull the gear assy. off of the shaft. This assy. will be the drive gear and a nylon hub. You'll very likley find that the teeth on the inside of the nylon hub have been worn out. I then went online and ordered the parts from PartsTree.com. You will need the Model and Serial # of your saw to look up the parts. Install the new parts,(very simple) and follow these directions in reverse, and you should be ready to go. I hope this helps.......
Tim
August 2007