2000 Rover 25 sluggish car

My car has started to run sluggish. Taken it to the garage they though it might be spark plugs, lead and coils. Two of these parts have been replaced and there is no change. The guy at the garage now thinks it may have something to do with the car not sensoring that petrol is going through and that it may be a technical fault. Rather than replace the coil which would be more costly he has suggested replacing a switch. Is this likely to be the cause, bearing in mind it just keeps backfiring and doesn't pick up speed even with the foot on the gas pedal?