I have a Giant STP1 (circa '05), with the Shimano derailleurs. Got a new Shimano HG40 chain to replace the old one (don't remember the brand/type), and it's skipping under load.
Derailleur? Skipping happens on most gears, but the derailleur is aligned, and straight. And shifting is precise, with no drop-out in any gear.
Wrong chain type? I don't have the original chain anymore (it rusted), but there was no skipping before, so I'm blaming the new chain. Later versions of the bike use the HG40 chain, but they use different derailleurs, and have a different cassette.
Does anyone know what the original chain was on the '05 STP1? The name Rohloff comes to mind, but I'm not sure.
Update: After reading some similar questions, I'm thinking that maybe the gears are worn out after all... But so was the old chain, and they fit together.