New SRAM SX slipping Just got my kid a new stumpjumper for Christmas.  It literally has 10 minutes of rising on it.  Noticed today on its maiden voyage that in the hardest gear it would clank like it was slipping tooth or something.  I can't make it happen in any other gear.  Any thoughts on what it might be, and how to fix it?. Guessing it has something to do with tuning.