I'm here again, just to say that now I'm able to use at full capabilities my rt2500 chip based wi-fi usb dongle
There was two problems:
1) Regression of the driver to a situation that I myself had tracked almost a couple of years ago viewtopic.php?f=5&t=6043
Old iwconfig power off solution still work.
2) Second problem was the invalid misc. I changed transmission channel from my router settings, and now, after 8 hours of uptime, I have just 73 packets dropped (on channel 1, after few minutes I had many thousands errors of that kind)
The last seems not to be imputable to driver itself, but with ndiswrapper this problem doesn't exists (but various kernel panics if there is high upload), so in my ignorance I think rt2500usb driver is too much sensitive to disturbs.
Thanks anyway to everybody