Regrettably I'm going to have to park the 737-800/900 in the hangar for the time being.It just doesn't want to operate properly on my Win XP system.Its predeccessor the 600/700 worked a treat and I've had many happy hours flying it around the virtual world.But its big brother seems to contain some strange gremlins which have got me beat and my patience has run out. I am operating it according to the manual and in most respects exactly the same as the 600/700.However, it has a mind of its own.First of all it was the V-nav mode which disengaged without warning during climb, cruise or whenever it felt like it.Occasionally it would work like a charm but then it would throw a wobbly.The other day on a flight from Cardiff to Salzburg it threw a tantrum about half way through the flight and the aircraft started plummetting earthwards.Now it's the L-nav mode which is starting to disengage without warning.It did this today on a flight from Glasgow to Cardiff at cruise altitude.I managed to re-engage it for a few minutes before it once again disengaged and the aircraft went into a diving turn.Heaven knows what's going on here.I haven't altered my system, added any new programmes or done anything to change things since installing the 800/900.All I know is that the 600/700 worked like a charm in all modes but the newcomer is unpredictable to say the least.It's a pity really because I think it's potentially a great product but its got me licked.Judging from some of the posts on this forum I suspect others may also be tearing their hair out.Good luck guys.

Too bad you don't supply us with the information we need to help you though, Frank.You can't fly this one as you flew the -600/700. First of all the 800/900 are longer and heavier. That makes a lot of difference in handling. Also there are updates to the AFDS that requires some new techniques from the squishy pilot.VNAV never disengages without a reason. Any alt/speed constraints that will throw VNAV on it's end? 6