On my 2001 Diamondback Outlook I noticed some clunk sound and slipping when pedaling. It happens only when starting up from standstill or on hills. If I pedal gently it will not slip though so only under load. At first I though it was the chain skipping or the derailer out of adjustment but I checked it and its ok. This bike is ridden in all kinds of terrain from street to off road. The cassette/freewheel is threaded onto the wheel so its a one piece design. I did try to put some lube on the part between the cassette and hub but didn't help. This bike is all stock and I didn't change or upgrade anything. Any suggestions and help, thanks.