For the second time in a roll, trying to set things up for a flight, just when I'm about to input the Cost Index, as I click the LSK for insertion of the value, FSX hangs - with the "closed hand" mouse icon all the way from there, I mean, FSX is just frozen, but not other applications. It's scary because I've never seen or read about this before, and today it just happened twice!

Is this reproducible? If so give us the details (route etc) and we'll try it with the SP1 beta.

Yes, I was able to reproduce it, as I tried it all again and it just happend for the second time. The way I use to fly is ... land, taxi, park, shutdown and then save the flight. Next flight (another day...) I would continue from there, loading that very same flight. Prior to experiencing this issue, I noticed that the flight plan/route wouldn't vanish from the FMC, like it always used to, it was like 2 flights before I started noticing this. I don't know if it has something to do, but...

From what I can remember, I was in KORD (FSDT) trying to setup a flight to CYUL (FlyTampa), then when I was about to insert cost index the problem arised. But now I could solve it by reseting the flight, I just loaded the plane from scratch and then I was able to perform all the steps as expected.

A couple of months ago I had such a similar strange problem, the other livery I was flying just stuck with the FPV on, no matter how much I clicked the FPV button, it wouldn't turn it off. I could just solve it by loading the plane again from the planes menu.