Ford Focus poor running

Whilst travelling at a steady speed with low revs the car momentarily 'misses', the revs do not appear to drop. I have had the car looked at and the Ignition coil, lead set and plugs changed but to no avail. In discussions with three seperate garages they cannot find out the problem as it has never happened whilst they have had the vehicle. I have also stopped buying supermarket fuel and keeping the tank above half, which appeared to help as I have not experienced the problem for a few weeks, but it has now come back. It does not happen all the time and is only momentary when it happens but it is as if there is no power or the brakes have been pressed lightly holding the car back. I had a full service back in March where all the normal things were checked/changed including the air and fuel filters.
Can anybody suggest anything else to try or if they have had similar problems?