I have this problem. There's some kind of short in the proximity sensor. Mine started like yours, but then quickly got so bad that as soon as you start a phone call, it will turn off the screen, like the proximity sensor is always in the on or off state (whichever activates/deactivates the phone). There's nothing I can do but constantly hit the power button repeatedly while trying desperately to hit one of the phone buttons; if I manage to hit the speaker button, then the screen will stay on and I can use the bloody phone. Hanging up involves hitting the power button repeatedly and attempting to hit the end call button until I finally get it to hang up - or the other person hangs up first.
I cannot get it repair under warranty because I got the phone at Build and the phone, according to Nokia's website is not covered in either Canada or the USA.
It is definitely a short however, as sometimes if I squeeze the phone in the right spot, the proximity sensor will work for a while. It's become rarer and rarer that it will actually work like that, however. So there must be a loose solder point or something. I'm eventually going to crack it open and attempt to fix it myself.