OP problem

[Cratos]

it seems after runnung down the X-way for about 10 mines whenI get off the ramp for the light my Op drops off to 12 and slowley comes back to 20 ( where it normanly runs hot @ idle)AND THE RPM DROPS TO 500 AND THEN BACK UP WHAT WOULD CAUSE THIS?? could a sticking IPR do it or do i need to start looking for a motor or new truck?@ 2000 rpm hot the op is still @ 38 like always.