Cables are cheap and should be inspected once in a while. I would remove the cable and check that there are no kinks in the housing first. Also, cables can fray inside of the housing, make sure the cable moves freely. Cable lubers are cheap too. Another potential is that if you are running hand guards, dirt and crap can find it's way into and under the throttle tube and the handle bar. Make sure this is clean and then relubed. I do this exercise on every bike I have had especially after a muddy ride. Another thing to look at is routing of the cable. Turn the handle bars all the way lock to lock and check to see if you still have the recommended slack in the throttle cable at both extremes. If it gets tight, the routing is probably not correct. Another thing I have seen is where hand guards were installed and not enough of the grip was cut making the grip rub on the guard. Look at the entire system and get in the mode of checking out everything often. Dirt bikes will take tons of abuse but do need to be maintained after every ride. My 2 cents.