HELP - Aircraft Tucks Below G/S

Recently all of my FS 2004 aircraft tuck below the G/S on an ILS approach. The aircraft intercepts OK but after the OM they start slowly going below and if left alone the aircraft will hit the ground prior to the runway. It's as though the ground is below the scenery ground being displayed! Can anyone explain how to correct this?

Ummm check altimeter is set??

If you do not have the altimeter set correctly, that will happen. Also, I have found some add on craft will do this. I do not know why unless the configuration file was not set up right. But, usually, it is not setting the altimeter correctly. I always hit the "B" key when I put my gear down just to make sure.