Fsbuild-2 Flight Planner

I have successfully installed ISG1 gauges in two aircraft: AeroSim Boeing 787 and AlphaSim C-5 Galaxy, mostly as a result of the retrofits and examples on the ISG website.

I have installed a new autopilot in the AlphaSim C-5 (taken from the Virtavia C-17 Globemaster), in addition to a Boeing 777 PFD and Boeing 737 ND and VNAV and LNAV buttons from the ISG1 gauges. With these installed, I can fly a flight plan sucesssfully using these buttons and gauges in conjunction with the autopilot. When I reach the TD point (top of descent) and don't intervene manually, the aircraft does initiate a descent but the autopilot sets the altitude at 300 feet. If I set the MCP altitude at 2000 feet, for example, in advance of the TD point (as is the case in some addons), then the aircraft does initiate a descent, but at the next waypoint, the autopilot altitude is again set to 300 feet. It would appear that the VNAV is not following the prescribed altitudes in the flight plan during the descent.

Is this a limitation of expecting too much from modifying freeware and expect it to work like a PMDG product, or are there means by which I can control (other than by manual intervention) the autopilot altitude setting in the descent, but still with VNAV and LNAV functioning? Ideally I would like to do what is more typically the case and set the MCP altitude to 2000 or 3000 feet and expect that the autopilot retains this setting until e.g. glideslope capture.