Vauxhall Astra losing power for short moment when going over 40 mph

For the last 4 weeks, I have been losing power, it feels as if I am losing it for a second or 2, seems to be when the car is hotter, and when this happens it feels juddery and smells more of petrol, and seem to use more petrol, the engine management normally comes on too, It has been in the garage several times, over the last few weeks, and on the diagnostic machine, at first I had the coil pack changed and new spark plugs, but still continued, it has come up on the diagnostic machine there is a fault with the o2 sensor, but when this was checked it came up as ok, also has come up on the diagnostic machine that it is misfiring, and air and petrol is not at right quantities, seems to come up with something different each time, when it is reset on the diagnostic machine it runs ok for a while then it starts again, wondered if any one had any ideas as Im at my wits end with it and even the garage, don't know what to do with it.