Low rpms...I use it to commute to work in the the city(with their poorly maintained roads), so I normally don't hit the throttle hard unless I'm passing someone.
Ok that could very well be from carbon deposits inside the combustion chambers then. That's a common problem when engines are driven too conservatively.
As a kid I saw a Corvette mechanic at a Chevy dealership repeatedly revving a big high-performance Corvette engine to redline and back down. Up & down. Crap was blowing out the exhaust as he did so. I asked why he was doing that and he bitched that Corvette owners regularly brought their cars in because they weren't running well and they had engine "run-on" as you described. The mechanic bitched that many Corvette owners were afraid to drive them as they were meant to be so carbon build-up was the result. Revving them hard to blow the carbon out was the cure, he said that was usually all he had to do to make them run well again.
So try this... it's called an "Italian Tune-Up" (Google it). Over no less than several days, when it's safe to do so of course, start driving your engine as aggressively as you can. Repeatedly revving the engine hard under wide-open-throttle and take it up to or at least near redline, like when getting onto the highway or freeway. Let the engine rev high and hard to blow out any possible carbon deposits. See if after a week or so if your symptoms don't get noticeably better or even completely go away.
It's of course not required to do that all the time but it's actually better for the engine if you occasionally push the engine hard with hard acceleration to high RPMs. That keeps it cleaned out inside.
One more thing that can help do it even more quickly. And yes I know what I'm about to describe sounds absolutely crazy like it'd cause hydro lock. But it's not crazy, it works, and done as described it gets rid of the carbon and cleans the combustion chambers. It was discovered during WWII by aircraft engine mechanics. Water injection. Yep, water. To develop more engine power to help get bomb-laden aircraft off the ground more quickly, water injection was added... which is still used to this day in some aircraft. The mechanics noticed that after the water injection was added that the combustion chambers were staying sparkling clean. No carbon deposits.
Nope you don't have to add water injection. What you do is this.... fill a 12 to 16 ounce container with tap water. Like a 16 ounce aluminum soft drink can. Remove the air intake from the throttle body. Start the engine and VERY CAREFULLY and SLOWLY start trickling the water into the throttle body while keeping the engine RPMs up with the throttle lever. Slowly enough that it should take no less than a couple minutes to drain the container. Maintain careful control over the can so you don't accidentally dump too much water at once into the throttle body.
What this will do is create steam which will create very small mini-shock waves inside of the combustion chambers (that area between the piston and the head) which will break up the carbon deposits which will be blown out through the exhaust.
Nope I'm not crazy, this is done all the time at most garages or dealerships. At least experienced mechanics will do it. It's commonly done because so many people drive their cars too conservatively thinking it's better for the engine. It's not.
