2006 Corsa C - 1.2 Twinport Intermittent Power Drop

On the motorway when cruising, the revs will rocket up and the power drops (as though I am suddenly flooring it in neutral) and I gradually slow, after about 30 seconds it rights itself. The garage told me it was the clutch, I believed them and paid for a replacement, but it hasn't sorted it! Any ideas would be appreciated, so I can be armed when I go back to challenge the garage. Thanks.