I have a hardtail MTB and just replaced my chain, which I broke on the trail. I bought a KMC X9 chain with the quick link and installed it myself. Shifting works fine and everything is smooth...until I really hammer down on the pedals. In any situation when I'm really mashing down on the pedals, the chain will jump/skip and make a loud CLONK! sound. Any idea what could be causing this?

For information about the bike:
- Windsor Cliff 29er hard tail MTB
- SRAM X-7 9 speed F/R Deraillers
- Truvativ 5D triple crankset
- SRAM PC950 11-34T cassette

The bike is fairly new, bought in April 2010. I've put around 500 miles on it, almost all of those being pretty rough trail miles. The chainrings and cassette do not look worn upon inspection.