It could be your freewheel/casette slipping. You are either gonna need a new freewheel, cassette, or hub (I think those are the right terms)...
Other than that, i'm not sure what could cause the problem. You might want to take it back to that bike shop, and ask them what the problem is. Most bike shops don't ''test ride'' the bikes, or if they do, it is just brief, nothing that might cause wear and tear, such as accelerating hard in high gears.
If you feel the chain slipping, take a look @ it, try to see where the chain is slipping, then relay that information to the bike shop you took it to, i'm sure they'd be glad to take a look at it for free
Oh one more thing... It could be the derailler slipping off, ask them to take a look at that too.
Last edited by rushlink; 09-06-07 at 09:52 PM.
Reason: added info