2004 MG ZR

Engine Performance problem
2004 MG ZR 4 cyl Manual 66000 miles

i have recently been experiencing problems with my car by it keeps losing power while I'm driving, it is as tho either then fuel ' is get stuck/starved ' or ' the fumes that come out of the exhaust are being blocked' this is causeing the car to lose power, but if I put my foot down for around 10-20 seconds it pass as tho one of the above is being released/giving way. I have had a new exhaust fitted this is when I first noticed the problem but it has started happening a lot more now. Its guaranteed to happen after I've put fuel in and pulling out of the garage and when I'm pulling out of junctions. I've taken it to a local garage where they have replaced both coils, and all four spark plugs. They originally thought that I had blown the gasket but that is fine. What else could this be please help.
October 15, 2010.

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