Does it do it with the throttle closed? If the engine rpm is rising, could you have an air leak into the system past the throttles? Like a loose inlet manifold for example? On my first ever car, I changed the downpipes and you had to take the inlet manifold off as well. In my haste to get home, I forgot to tighten the manifold bolts up and when I started the engine, the revs were all over the place, up, down , almost redlining. Just a thought.