Another oddity . . . the chronometer is correct at the start of the flight, but "loses time" relative to the system clock (and the online clock) as the flight progresses . . . this obviously affects the FMC also . . . I know my PC is a bit underpowered, but is that going to be the cause . . . why is the clock "unlocked" from the system clock or is that an MSFS thing?

It's an MSFS thing. Hopefully in FSX they will of used their brains and made a DLL that goes in the modules folder to sync with the computer clock, i mean its not difficult to do as there is several clock add ons that do it, there is an option in the FSUIPC dll(if you have it) that syncs your FS clock with the main clock but it only syncs it second by second so if you pause the game or something you will still be off time but not off "tick" if you know what i mean...

I believe registered FSUIPC has an option that will fix this...

You might also try FS Real Time, a program designed to keep the FS clock in synch with PC time, latest version(I think!) available here:http://www.fsdome.com/library/fs98tools/1.htmFrank