Thanks for the reply Ben, I'll give that another try, but I'm sure I've already tried that.
I see that a lot of people appear to have that problem "sometimes", but for me it's "all the time!!"
I'm waiting for the developers to come back from holiday and perhaps be able to help me with it. I reinstalled fsx last week and tried again and it worked perfectly for 3 or 4 times, but since then it is not working at all and I crash out every time. I've had to totally abandon that plane for the time being as it's no fun trying to fly without the FMC setup.
the worrying thing for me, is that this bug, despite being mentioned by many people, is not even mentioned on Wilco or FeelThere's sites as being a known bug. They have a "solution" posted on Wilco's site, but it only pertains to FS2004 version.