Help to Diagnose a Problem with Northrock XC6 Mountain Bike I have been fixing up a Northrock XC6 bicycle.

Symptoms:

Chain clunk when trying to start moving. It feels like the chain is slipping. It only happens under load, I cannot get the same symptom by running the wheel by itself. This makes it impossible to get started. However, the same problem does not occur when at higher speeds.

Background:

I just installed a new derailleur, new chain, and new pedals. I needed a new derailluer because a similar problm happened and bent the old one and broke the chain. 

What is wrong with my bike? Thanks!
Cassette sprockets are highly worn and chain is slipping off of sprocket teeth, or (and more likely I think) the freehub freewheel mechanism is slipping, either due to pawls not extending properly or damage to pawls or body.

Sometimes freewheel mechanisms get gummed up with dried out grease which makes the pawls stick. Liberally applying solvent can sometimes unstick them, but I'd look into disassembly and regreasing.