I think my proximity sensor gone bad

I owned my 32GB 3GS for about 10 months now. it used to be great initially but after 6 months of use it started having an intermittent problem. Basically it would flickers black screen 3 times once I start a call and automatically shuts off the scree even if it is not on my ears. I checked similar problems on the web and thought it was my old phone case (2G bulky case). So I removed the case and restored the phone. it seemed to be working for few hours then again it happened. it worked intermittently for 2 months then now the problem is persistent.

I am suspecting the proximity sensor because:

1 - it flickers always 3 times before it happened (it maybe a warning thing).

2 - it does not plank the screen when I manage to race to the speaker button before the call is connected.

3 - it does not plank the screen when I use any hand free device.

Please advise if my suspicion is valid and how can I fix it?

PS - I bought the phone officially unlocked when I was in Australia and now I am in the middle east. So warranty is not a good choice.