Join Date: Jul 2006
Location: Sammamish, WA
Bikes: Cannondale Prophet, Specialized S-Works SL2, Specialized S-Works Stumpjumper
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Since the chain is only skipping in those two cogs and since they're smaller and require more of a bend in your chain I would start there. Look for links in the chain that bind or don't move as freely as the others. If you can't find anything, flush the chain with lube and work the links to get the lube deep into the pivots. While you're at it check those cogs for obvious signs of wear. Shift the chain to those cogs and see how it fits. You could have worn cogs or a worn chain. If the derailleur shifts normally through the entire cassette I doubt that's causing your problem but if you can't find the cause of the chain skipping you may want to check its alignment.
Unfortunately, I know nothing about that particular freewheel so I don't have any recommendations about a replacement.