Hello I have a problems with the gsm signal.

I have a possible solution, in a nokia n95 I improved the gps signal welding a headphone cable into n95 in a chip gps zone and gps improved, better signal and I could plugged sattelites long before

In a n900 antenna zone welding a copper cable /headphone for example,
Do It will can to get a better signal???

GSM antenna (and wifi for that matter) is excellent on the N900. Without any further details, I guess it's therefore likely to be the issue stated here:

http://talk.maemo.org/showthread.php?t=77248

Solved mainly by either reballing or employing a deviation of the old Xbox "penny fix".

Or, in my case, sending my N900 back to Nokia while it was still under warranty.

My n900 recognize my sim ok, the problem is signal power , hight-low every-time.
I live in a big city and power signal not problem because with other mobiles galaxy s for example I had very good power signal every-time, now I will read the thread that you recommended to me.

Thanks

For sure, I had the same issues. At no point did I have SIM problems. It manifested itself in the same manner as you described and progressively got worse, to the point where moving from one cell to another caused the signal to drop altogether (and incredible battery drain).