Stop loading saved flights and try it using a default NGX panel state. If the problem goes away which I am willing to bet it will, then you know its FSUIPC autosave. Example: Before SP1 and with Hotfix 4 I had the following issues. If at cruise and for whatever reason FSX crashed and I came back and loaded from my last FSUIPC save VNAV was totally inoperable the rest of the flight. This was fixed in SP1 I guess as it no longer occurs. But the best way to troubleshoot any issues is to start by using a clean default panel state and work from there.

Check Alt, or altitude for short indicates that adjustments need to be made set altitudes in the FMC. I usually get check alt if I won't be able to make an altitude restriction for some reason or another. Also unrelated but useful when your set to ascend or descend and your mcp altitude needs to be reset. You will receive a warning sound from the autopilot, your FMC message button lights, and on the FMC it reads "Reset Mcp Alt". That or something similar anyway.Still onto the other issues at hand, if your not inserting changes regarding speed/altitude in your plan then you should see not as many warnings. In combination with the check alt fmc message your getting I believe I have an idea of whats wrong.FCOM Vol 2 page 4.10.6 is the reference.VNAV mode is terminated by any one of the following:•selecting another pitch mode•glideslope capture•reaching end of LNAV route•transition of glideslope intercept waypoint if G/S is armed•crosstrack deviation exceeds twice the RNP value during PTH descent for an active leg with a database vertical angle and LNAV not engagedIn the event of glideslope intercept waypoint transition, VNAV can be re-engaged.When watching the video I noticed your FMC altitudes all reset to 0. This is what caused the Vnav to disarm itself, which is not abnormal in anyway. However, the way your FMC reset all the altitudes is strange, make sure your not using in faulty panel configurations, and if you don't have it already, download Sp1b.

Thank you for your input guys,I do have Sp1b installed, and I think that the saved file must be corrupted. I will try again without the use of saved files.Again thanks to you all!