BrianG wrote on Jul 2^nd, 2012 at 4:43am:

DT, I used to have that problem as well. By accident I found the cause was running the AP in Nav mode when using the Doppler or GPS. If I disengage the AP to land by hand while the AP was still in NAV mode, my GS would act exactly as you described. I found the solution was to flip it from Nav Mode to LOC/VOR before you disengage the AP. Then the GS would come alive and function as it should. Auto landings worked fine. It was just failing to go from Nav to LOC/VOR before turning off the AP for a manual landing is where the problems would start

That was exactly the problem and like you said solution was to go to LOC/VOR before disengaging AP. Thank you.