I had this issue with previous FW, but i had installed testing/develop stuff. Rebooting it every day fixed it for day and finally i flashed fiasco+vanilla and it fixed issue.

I flashed new PR 1.2 and the problem persist. It's VERY annoying!

both images?

.edit
after flash did you install anything?

One image: "combined" that inculdes all firwmare components.

Yes, i have installed almost all previuous software via backup list.

What is the software that generate this issue?

Dear god ....

First thing what i would suspect is of course some widget/protocol/app that i have installed. Run phone for one week without any installed stuff and see how it works.
(this is price what you "have" to pay when you do not have strictly controlled environment. Extras is probably safe, but still there is apps that could end up eating memory or start swapping)

What is combined image?I´m quite sure that fiasco and vanilla are separate images. Did you flash with flasher or with nokia software updater?

I used flasher via command line. And flashed only one bin.

This is definitely a hardware issue which is not caused by any extra software. You can check whether your N900 suffers from this issue or not by entering this command in Terminal:

You should really get "open" when nothing is covering the sensor. I personally always get "closed" when I am outside, which means my N900s proximity sensor is probably defective.

If you also experience this behaviour, then please vote for bug #7772 on bugzilla (https://bugs.maemo.org/show_bug.cgi?id=7772). Please remember not to use bugzilla as a forum - adding a vote is completely sufficient to show that you are also affected by the bug.

My phone is on PR1.3. The problem persists. Does anyone's phone (still) has this problem?
The case is still open in the bugtracker
https://bugs.maemo.org/show_bug.cgi?id=7772
I fully agree that it is very annoying. Its always takes me a while to answer calls. I find it ridiculous to talk with the keyboard open.
Anyone?

This is a hardware issue which early revisions of the N900 have. Take your phone to the next Nokia Care service point and have the proximity sensor replaced on warranty. I did that with my phone - the service guys were informed about that issue by Nokia and they've fixed this within one day without any complaints.

Thanks for your advice. Do you mean replace the phone or just the sensor?