From what you have described, it sounds similar to how my RST went when I put a new head on it. My Spark plugs were shot, just popped some new ones in & it was as good as gold.

Clean the plugs off & try driving it rather than giving it half throttle for a view minutes.

What plugs are you using?

Make sure the car is set to all the std timing marks or it won't run right.

Dan