Running it too low and it is sitting on the bumpstops would be my guess
Saw a Corsa on the way home with the front on the bump stops.
I thought it was a funny as feck... I obviously didn't control the look on my face as the driver clocked me... He didn't think it was that funny...

A nice moment after a hell of a day at work...
P.S. Sorry to hijack your thread... Did you check the spring rates? They should be about stock for working suspension - lowering needs stiffer shocks to reduce travel. If the spring rate is too high it will make the car 'bumpy'.