Bad Signal in Access Point with hostpad 1.0 and USB Wi-Fi Adapter with Ralink chipset

Hi, I have setup one Access Point with Linux Debian on a GuruPlug Standard computer from Globalscale Technologies.

I am using the EW-7711USn USB Wireless adapter from EDIMAX (Ralink chipset) with hostapd 1.0.
The Access Point function is rather stable but, unfortunately, the radio signal is not stable and it apparently fluctuates with the time.  Because of this problem the useful range of the Access Point is just a few meters away.

By suspect is that this issue is caused by problems in the “rt2800usb” driver.
Below you can find details on my system.
Can you please help me ?
Thank you
Giovanni

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These are some details on my system:
cat /etc/issue
Debian GNU/Linux 6.0 \n \l

uname -a
Linux dreamplug-debian 3.4.6 #6 Wed Aug 8 02:00:27 EDT 2012 armv5tel GNU/Linux

lsusb
Bus 001 Device 003: ID 7392:7711 Edimax Technology Co., Ltd EW-7711UTn nLite Wireless Adapter [Ralink RT2870]
Bus 001 Device 002: ID 1a40:0101 TERMINUS TECHNOLOGY INC. USB-2.0 4-Port HUB
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

lsmod |grep -e rt2 -e rt3
rt2800usb              11669  0
rt2800lib              46246  1 rt2800usb
rt2x00usb               8242  1 rt2800usb
rt2x00lib              34777  3 rt2800usb,rt2800lib,rt2x00usb

modinfo rt2800usb
filename:       /lib/modules/3.4.6/kernel/drivers/net/wireless/rt2x00/rt2800usb.ko
license:        GPL
firmware:       rt2870.bin
description:    Ralink RT2800 USB Wireless LAN driver.
version:        2.3.0
author:         http://rt2x00.serialmonkey.com
srcversion:     62996BD74CC1C1E1D499866
alias:          usb:v0586p343Ed*dc*dsc*dp*ic*isc*ip*
….
alias:          usb:v07B8p2870d*dc*dsc*dp*ic*isc*ip*
depends:        rt2x00usb,rt2x00lib,rt2800lib
intree:         Y
vermagic:       3.4.6 mod_unload modversions ARMv5
parm:           nohwcrypt:Disable hardware encryption. (bool)

modinfo rt2800lib
filename:       /lib/modules/3.4.6/kernel/drivers/net/wireless/rt2x00/rt2800lib.ko
license:        GPL
description:    Ralink RT2800 library
version:        2.3.0
author:         http://rt2x00.serialmonkey.com, Bartlomiej Zolnierkiewicz
srcversion:     124CCC2A3BC2D92C51C034B
depends:        rt2x00lib
intree:         Y
vermagic:       3.4.6 mod_unload modversions ARMv5

modinfo rt2x00usb
filename:       /lib/modules/3.4.6/kernel/drivers/net/wireless/rt2x00/rt2x00usb.ko
license:        GPL
description:    rt2x00 usb library
version:        2.3.0
author:         http://rt2x00.serialmonkey.com
srcversion:     9177BE9E451CF386B810F3D
depends:        rt2x00lib
intree:         Y
vermagic:       3.4.6 mod_unload modversions ARMv5

modinfo rt2x00lib
filename:       /lib/modules/3.4.6/kernel/drivers/net/wireless/rt2x00/rt2x00lib.ko
license:        GPL
description:    rt2x00 library
version:        2.3.0
author:         http://rt2x00.serialmonkey.com
srcversion:     F43E854857739546AE9D67D
depends:
intree:         Y
vermagic:       3.4.6 mod_unload modversions ARMv5