If you haven't already. Throw a full tune-up on the engine. When changing the plugs do 2 things -- read them for a rich or lean condition and make necessay adjustments to the fuel delivery system -- second run a compression and leak down test check on each cylinder.
I'm assuming that since you are posting on this site your vehicle is non-computer. However, it may still have an air pump and PCV valve -- check those for proper operation
