I have this same problem aes well, since I bought it.. and my unit is not defected (dust in the sensor), It seems to be software or something problem from what I've read and it's a common problem..
I saw one ''tutorial'' to fix it and it actually helped, wait for the night, go to your room (a very dark room, more dark = better), put your finger in the sensor and soft reset the phone.. it helped me, now it flickers much much less and with much less intensity as well..
920's screen flickers in some situations. Are there any fix for it?
It's a known screen defect - get it exchanged. Had that on one of mine as well.
You can always download the Care suit and run your phone in Test mode, then recalibrate your proximity sensor.. Works for me!