I have a 2006 XR650R that has about 1,200 miles and has ran perfectly until now. I recently had a Trailtech Vapor installed as well as a Clarke 4.3 gallon tank. The first ride out it ran perfect as usual. Then I began to notice about 15 miles into my second ride the engine begin to stutter like it was going to die but didn't and continued to run after I closed the throttle. This occurred as I was riding on level ground, a fire road, at about 20 miles an hour in second gear. I have a Baja Kit installed so I usually ride about 10 to 15 miles on the road in route to the woods and this is where I have began to notice the bike stuttering progressively more on the second ride so I changed the plug as it looked dark and sooty. On the third and fourth rides it has continued to behave the same; engine stuttering around 15 to 20 miles an hour, mostly second gear, sometimes first and rarely third. As soon as I let off the gas for a second or two and roll back on the throttle it seems to run fine again. When it continues to stutter I shift up to the next gear and it runs fine again. It never dies and starts easy. It is not the plug, not bad gas, and no the choke is not on. What do you think?