Since I installed the NAV/GPS button modification in my 727 I have found I can't use the AP knob (the 2D) to control climb and descent.  As soon as I engage the autopilot paddles during a climb the plane immediately goes into a 2 to 3K fps dive.  I keep clicking the 6 o'clock position on the knob and at first the angle becomes less steep but additional inputs have no effect.  The opposite happens during a descent, I can't control the rate and I have to disengage the AP and trim for the climbs and descents.  
Any ideas about how I can correct this problem?