If you've checked the plugs, and it's still missing, then it could be something to do with your internal combustion timing and exhaust manifold gear differential turbulence...
For example, if you've got a plugged sphincter valve it will build up the warm exhaust gasses slowly over time until it pressurizes enough to be forced out... this will cause a miss accompanied potentially by a loud backfire.
Of course, this will also do a number on your catalytic converter.. Do you by chance smell rotting eggs after an exhaust discharge? if so, the catalytic converter has already started it's decomposition process and will have to be replaced before your next inspection.... they are kind of expensive..
if you smell raw unburned gas from the tail pipe, that's another sign. as well as any blue smoke, or any watery substances leaking out.
definitely take it into the shop and ask them to check your sphincter valve with a stethoscope... they have specially designed ones that they use to listen for engine knocking in specific places etc .. it has a long rod on it that they insert into various ports on the engine to pinpoint noises.....
They will gladly insert it in your exhaust manifold and check your sphincter valve for leaks or plugs. a new sphincter valve is typically 15 or 20 bucks at auto zone..... you could probably install it yourself if you needed to. a shop would probably charge you 50 for the entire thing labor and all
