Ford Focus engine hunting when cruising

My car was Out of Warranty in October, 2008.

Just before the Warranty finished it went in for drivability problems.

At a constant speed the engine was hunting i.e. slowing and picking up again.

At tick-over and when accelerating it was alright.

It was put on test and I was told that no faults were found.

It has since been doing this fault intermittently.

Just before Easter it did it quite badly and has been doing it since.

I took it in for a test drive at Sandicliffe, Leicester, but it did not do it for them.

It went in for diagnostic checks on 6 May, 2009 and again they have said no fault was found.

I drove it away and it has since done it again.

During Warranty it has had a new alternator, two coil packs and a coil loom.

I am not happy with the situation and welcome your comments.