<<<I have been looking at other sound editing programs - I really LIKE audacity, and I am used to it, but if I can't resolve this issue, it's kind of pointless to continue with it.>>>
Post back if you find something you like.
<<<Thus if you delete/move/modify the original file,
the content silently dissapears from audacity, too.>>>
Yes, we have visited this town already. The poster is not moving any files, although he gets dangerously close to admitting he does here and there, and moving things is a major cause of Audacity Insanity.
<<<I've saved the .AUP file >>>
That's dangerous. Audacity doesn't save AUP files. It saves 'Projects" and the lead file of hundreds in an Audacity Project is the AUP file.
I get the feeling that we are reading the 8 volume set called "How To Ride A Bicycle." If you started in the middle of any of the eight volumes and start reading cold, you would never figure out what the end result needed to be. So let's go back to basic engineering.
Do you have a System Drive or any other drive on your machine that's over 90% full? Doesn't matter if you're using it for the project or not--any drive the machine can see.
Do you run an anti-virus software? Have you done a comprehensive system scan--the one where you can't use your machine for several hours?
<<<Bugger. It's done it again.>>>
I swear this is a known problem, but I can't put my finger on it.
<<<The other stupid part is it didn't do this from moment one - it has just recently developed this habit. - I used to lose a small sound effect here or there, but not entire tracks this way.>>>
So your machine has always been unstable, but it got worse.
I would clear the basic machine with the Virus Checker and then run this Microsoft memory checker overnight.
http://oca.microsoft.com/en/windiag.asp
I'm not kidding. Start it looping when you go to bed. Normal machines will do this for a week straight. I had two machine with bad memory that would go two hours under test and then crash. These were the machines that would "do funny things" every so often and drive me nuts. Sound familiar?
<<<iMac G5 running Mac OS 10.5.>>>
OK, so that machine is running Leopard. Is the PC running Vista? I don't know that we ever established that.
Koz