uneven running at 1000rpm?

i have a 1.2 sxi vauxhall corsa and recently noticed that if i hold the engine revs at a steady 1000rpm the revs will not settle it increases and decreases and you can feel an uneven running within the cockpit of the vehicle. an engine diagnostic check was conducted at my local vauxhall dealership but no faults were found or recorded, can anybody help!