laptop randomly drops network connection

Configuration is as follows: Dell Precision 4300, Windows Vista Ultimate 32bit, Intel wireless wifi link 4965AGN, Broadcom NetXtreme 57xx Gigabit controller. After power up internet connections works OK. But then randomly after few hours internet connection is broken. I run Wireshark and I see that there are no responses to the packets sent from the laptop to the outside. In Wireshark I see packets coming from outside just from the previously established connections and also I see packets sent to the outside, but with no reply to them.

Intel statistic for the wireless adapter shows me that IP packets are received (they are also showed in Wireshark), but no packets are sent to the adapter - although Wireshark shows that packets are sent.

Based on the above I presume that Vista network stack sent packets, packets are detected by Wireshark, but those packets do not reach the network adapter (neither wireless adapter nor wireline adapter, adapter send statistic remains unchanged) and of course are not sent to the outside. On the receiving side I presume that everything is OK. Adapter receive statistics increases and received packets are visible in Wireshark.

The behavior is the same both on the wireless and wireline adapter - both of them work for some time and then stops.

Scenario is as follows - I have wireless connection, wireline connector is unplugged. Everything works, then internet connection is dropped, after that when I plug in the wireline connector, connection is established over wireline again for some time until it also drops. Now the only solution is to reboot the laptop.

Using on/off wireless switch does not help. Also disabling/enabling/diagnosing adapter in Network Control panel is not possible. It is not possible to disable or diagnose the problematic adapter - Network Center just reports error. Disabling adapter is only possible when adaptor is still in the operational state.

Except the standard Vista Ultimate defender and firewall I do not have any other 3rd party firewall or virus protection SW.

Any help or suggestion?