My idle started acting weird recently and I can't figure it out. When I rev it up and drop it to idle, it idles at 1400 for a minute or so and then will drop to 900. I can't get that 500 rpm jump to clear out. I've plugged the vacuum lines, sprayed for vacuum leaks, replaced the throttle plate, removed the power valves, balanced the idle screws, reset the floats and ensured the timing IS NOT increasing at the higher idle. Also replace the plugs and wires and the compression and valve lash are perfect. I'm running a LARGE solid cam and have very low vacuum at idle but it was idling fine before now. Any ideas?