DM8168:Sending data using TCP by wireless LAN

Hi,
I want to send data using TCP by wireless LAN on USB 2.0 port of DM8168 board.
I can look see stable connection under 5 Mbps(send rate).
But sometimes, send rate is felled to a low of 0 Mbps when I tried to send data at 10 Mbps.
It is the same if I tried to send data using UDP at 15-20 Mbps.

I am on the following of connection.
---------------------------------------------------------------------------------------------
Linux kernel : linux-2.6.37-psp04.00.01.13.patch2
wireless LAN adapter : LAN-W150N/U2(Device : RT3070)
wireless LAN driver(source code) : 2011_0407_RT3070_RT3370_RT5370_RT5372_Linux_STA_V2.5.0.2_DPO
---------------------------------------------------------------------------------------------

It is the same when I made sure the throughput performance using iperf.
/****************** iperf Log ******************/
[ 3] 0.0- 1.0 sec 5.91 MBytes 49.5 Mbits/sec
[ 3] 1.0- 2.0 sec 5.06 MBytes 42.5 Mbits/sec
[ 3] 2.0- 3.0 sec 4.74 MBytes 39.8 Mbits/sec
[ 3] 3.0- 4.0 sec 0.00 Bytes 0.00 bits/sec
[ 3] 4.0- 5.0 sec 0.00 Bytes 0.00 bits/sec
[ 3] 5.0- 6.0 sec 0.00 Bytes 0.00 bits/sec
[ 3] 6.0- 7.0 sec 0.00 Bytes 0.00 bits/sec
[ 3] 7.0- 8.0 sec 920 KBytes 7.54 Mbits/sec
[ 3] 8.0- 9.0 sec 5.10 MBytes 42.8 Mbits/sec
[ 3] 9.0-10.0 sec 5.21 MBytes 43.7 Mbits/sec
/***********************************************/

However, send rate was not felled to a low of 0 Mbps, when I made sure the throughput performance using iperf with Linux PC, same wireless LAN adapter and wireless LAN driver's source code.
What's more, the average of the throughput performance is over 80 Mbps. (same network environment)

I've used ASIX 88772A based USB-Ethernet adapter to try same test.
Send rate was not felled to a low of 0 Mbps too.

When send rate is felled to a low of 0 Mbps, wireless LAN driver was read out the following of log message.

"BulkIn IRP Pending!!!"
or
"BulkOutDataPacket failed: ReasonCode=-2!".

When wireless LAN driver was read out "BulkIn IRP Pending!!!", RT3070's status is Rx FIFO full in status register.

I am troubled why send rate is felled to a low of 0 Mbps only DM8168 board.
Is there someone who have an experience with the same occurrence?

I want to send data using TCP by wireless LAN at 10 Mbps stable.
(case of taking no thought band frequency or packet loss)

Would you be able to help?

Best regards,