I used to suffer this bad until I discovered an inlet manifold leak. I fixed it and it was good for a weak or so and now it's not good at all. Manifold bolts are all still tight and no loss of coolant. However it's very flat and definately underperforming and I'd say a lower mpg (hard to tell though).
If I leave it to idle for a while then it seams it's trying to stall or something. I can hear he shudders of it, but the rev gauge sits fine... I pulled the two spark leads and tested and they are sparking, and spark plugs seam fine also. It doesn't sound like a total misfire just struggling to stay idle (but as I say rev gauge is fine).
Other small symptoms that may help: purge tank (black cylinder) is ticking. Electrics may be bad because when I have lights on and say lock the car or turn indicators on the cars lights purge with it (alternator tested fine to 14.1v when running). Also slight sucking noise from around where I fitted air filter but it is all tightened and throttle body bolts are tight, doesn't seam like anything is wrong to the eye....
Also slight whistle when reving the car above 3k rpm at idle (also noticable wen driving in 1st and 2nd)