any ideas on this one??!!!!!!!

hi guys,
everything on the pits working great,except on short final the aircraft loses the glideslope and starts to lose altitude of its own accord,the pm mcp is still on ,and works flawlessly before this event,glide slope and loc stil display fine on the pfd,on both captains and first officers display,im using fs2004 and the westwind 737,overall the aircraft flys like a dream so id rather keep using it,the default 737 latches to the glideslope and loc and rides them down with no problems at all,
hope someone can point me in the right direction,
ps,im working on my instructor station ive had a metal frame and stand made to mount two keyboards ect,
also ive completed a security frame and metal panels to protect my hardwork and hardware,
cheers lads,
steve