Dell :: Slow Wireless Connection With Vostro 3550?

Four weeks ago I bought a Dell Vostro 3550 notebook to use it at home. I conntected it via my wireless accesspoint to get access to data on my NAS, which is within my home network.Then I noticed that the connection is not stable, although the computer says "excellent signal". Whenever I access files via explorer on the NAS I loose connection from time to time and the reaction time is very slow (sometimes I have to wait seconds).During search for the reason I pinged directly the access point and saw unacceptable slow response with large variation. I checked the connection with an other computer (running XP), using same position. There it works fine (see below) - stable connection.It seems that the configuration of the access point does not work well with the configuration of the Dell computer - or an hardware fault on the Vostro. Below are the test data and my configuration.

testdata> ping Vostro -> accesspointPinging 192.168.2.120 with 32 bytes of data:....Ping statistics for 192.168.2.120:       Packets: Sent = 30, Received = 21, Lost = 9 (30% loss),Approximate round trip times in milli-seconds:       Minimum = 1ms, Maximum = 2439ms, Average = 348ms

[Code]....