ah oh, that sounds quite strange... i'm not normally aware of a clutch slipping, and then gripping... I do apologise if im teaching grandma to suck eggs, but, are you definitely fully engaging the clutch? the clutch is set quite high on a zed, which can cause problems when pulling away, for example. just wondering if you're back on the power before the clutch is fully engaged, it'll feel like it slips a bit.
as I say, best way to be sure is 5th gear at 70mph, clutch fully engaged (ie make sure you're not riding the clutch) and then floor the throttle. car will accelerate, but revs shouldn't spike at all.