747 Nav Problems

Well guys, my routing today was EGLL to OMDB. I have noticed the following issues:-
1, The FMC at various stages stops updating the next waypoint, you have to press the Legs key in order to update the waypoints.
2. Once the above occurs the Lnav ceases to function and track the flightpath, disarming the AP and reverting to heading select to correct your course to intercept, then pressing Lnav seems to work to correct this, but not for long.
3. When on Vnav and Lnav input of a lower flight level, i.e TOD, initiates an instant descent without pushing the flight level selector button.
4. FMC legs page totally dissappeared from the screen on descent.
Has anyone else experienced these issues, as it stands this model is totally unflyable !! .

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