AP Altitude Setting in Fokker 100

OK, my first problem with this plane. I'm sitting here at Boston, trying to dial in 31000 feet in the Altitude window, but it will not reset to zero. It's at some odd flight level like 32654 and I can't get it to just go to plane ole 100 intervals (if that makes sense).Anything I can do to fix it, otherwize I'll be flying at an incorrect altitude on VATSIM.thanks!