Seejay:

Try this: Go to the altitude at which you expect to intercept the glideslope, set the AP to Altitude Hold, then engage the G/S Auto position on the AP. ILS Lock will remain green and G/S lock will be yellow until intercept, at which time the Altitude Hold will desconnect and the G/S light turn green. The airplane will follow the slope correctly. For some reason, it pitches up slightly just before starting the descent.  

I've noticed that if you engage the G/S Auto position and continue to make manual changes in altitude, the airplane behaves as you described.