各种门事件下载网站:网络时有时无是什么原因？

我用的是ADSL上网，原来用的是中兴的猫831 时常出现断线，出现网络灯熄灭。在没有熄灭的情况下，显示有网络连接，但是什么也上不去。后改用了831Ⅱ 网络灯正常，没有灭过。但无法正常上网，电脑显示有网络连接，但还是什么也上不去，重启猫后可以上去，有时一会儿又上不了了 电脑显示有网络连接！原本用的是主板自带的网卡，也用过阿尔法8193网卡，问题还是一样。在显示有网络连接时有发包无收包，用诺顿网络特警2006（更新）和卡巴斯基（更新）杀过毒，重装过系统。请各位帮忙找找什么原因。
C:\Documents and Settings\txl3217>ping www.baidu.com
Pinging www.a.shifen.com [220.181.18.165] with 32 bytes of data:
Reply from 220.181.18.165: bytes=32 time=53ms TTL=50
Reply from 220.181.18.165: bytes=32 time=54ms TTL=51
Reply from 220.181.18.165: bytes=32 time=50ms TTL=51
Request timed out.

Ping statistics for 220.181.18.165:
Packets: Sent = 4, Received = 3, Lost = 1 (25% loss),
Approximate round trip times in milli-seconds:
Minimum = 50ms, Maximum = 54ms, Average = 52ms

C:\Documents and Settings\txl3217>ping 220.181.18.165

Pinging 220.181.18.165 with 32 bytes of data:

Reply from 220.181.18.165: bytes=32 time=51ms TTL=51
Request timed out.
Request timed out.
Reply from 220.181.18.165: bytes=32 time=51ms TTL=51
Ping statistics for 220.181.18.165:
Packets: Sent = 4, Received = 2, Lost = 2 (50% loss),
Approximate round trip times in milli-seconds:
Minimum = 51ms, Maximum = 51ms, Average = 51ms