Possibly your oil is foaming. Is the level ok?
Try holding the rpm at 3500 in neutral for 2 minutes, then back off the throttle quickly. If the engine then idles rough you have a high pressure oil issue. If the level is ok, remove the IPR valve and clean it.
Try holding the rpm at 3500 in neutral for 2 minutes, then back off the throttle quickly. If the engine then idles rough you have a high pressure oil issue. If the level is ok, remove the IPR valve and clean it.