If you've had a new exhaust system installed it may be that the exhaust header nuts need tightening. Generally they need to be snugged up a couple of times. Just go easy those buggers will snap off.
The other problem may be clogged slow jets on the cool side. Does it idle smoothly? If not a strong dose of Techron might fix you up. It's cheap enough and doesn't require wrenches and laying on your back like tightening the exhaust header will.
