How long since a tune-up? Any applicable mil codes? There isn't much load on the engine in park or neutral. There is load on the engine when you put it in gear. Another thought is to clean the throttle-body and idle air system.
You may need to do a cylinder compression test, to see what shape the engine is in?
December, 20, 2012 AT 11:29 PM
What size engine?
Check for a major vacuum leak (you'll hear the hissing sound underhoos) with the engine running in park. Especially look at the vacuum line coming from the brake booster to the intake manifold.