Problem with aircraft in FS2004!

When i attempt a flight in FS2004 i'm struggling to control the aircrafts pitch movements. Im using a flight sim yoke by ch products and everything is set to default but when i try to descend in the cessna for instance, there is a slight delay then the aircraft takes a dive then when i pull up to stabalize it, it starts to climb. it seems very hard to make small changes in pitch even using trim. I've also noticed this with add on aircraft such as the PMDG 737 on autopilot, the aircraft bobs up and down to achieve altitude. Never noticed this problem with FS2002, could there be a problem with the handling in 2004 or have i set things up wrong. Any help would be appreciated.K Farrow.