Help with a 2.3 Galaxy

Heres the problem, new clutch fitted 250 miles ago, now at 1,000 rpm I
get a knocking and it feels like the engine is going to cut out, the
mechanic said the clutch was good for a minimum of 200,000 miles but I
don't believe mechanics at the best of times anyone have any thoughts
on this. Oh when I checked the oil it was not far of dry!! when I set
off it was full.

Martin