I agree with [MENTION=3026]alcjphil[/MENTION] that a mismatch between a new chain and a worn freewheel/cassette is the most likely cause. An 8-speed chain should work fine on a 7-sprocket cluster, as long as both are in a similar wear state. Take the wheel in to your LBS and have them replace the freewheel with a new one of the same tooth counts (to avoid having to re-size the chain again).