Engine idles up and down with no rpm change, severly poor mileage too..its half!?

what are all the causes of this?? the rpm idles at 600 in park for example but i can hear slight erratic purging type of sound in the engine. sounds like quiet easy reving with a steady low rumble but the rpms dont change at all. or is this normal? im also getting half the gas mileage with no apparent reason what so ever. no scan codes come up ( but has hstory of a high number of multiple misfires) , newly tuned up, new oil change. new fuel filter too. the car seems sluggish getting up to normal city speed but after 60-80 kph it goes with no feel of power loss. hitting the gas pedal getting to 60 as slow as i can seems to push the pedal in a fair bit. any ideas?? no mechanic in 3 provinces can figure out why my car (2003 grand am se1 3.4l v6), is only getting 260km per tank and its not any of the usual suspects like filters, plugs, wires coils, this is a good one for you great ones out there who are looking for a challenge!!