The pcv (positive crankcase ventillation) valve is supposed to pull a slight vacuum on your valve cover so that oil vapors are drawn into the engine and burnt rather than creating pollution. If your valve cover, oil cap, or pcv grommet leak significantly, you can be drawing a fair bit of air (mixed with vaporized oil) into the engine. Check to see if you have oil in the line between the PCV valve and the throttle body. The slow down of your engine when you remove the oil cap is caused by loss of vacuum. This is normal. Warm the engine until it starts smoking. Turn it off and plug the line that goes to the pcv valve (socket extension works well) so that no vacuum is being lost to the throttle body. Restart the engine and see if it still smokes. If not, you're drawing too much air across your valves and pulling oil into your combustion chambers. See where the leak is. Your entire problem may be as simple as a bad valve cover gasket if it's really bad.