I have seen this issue on certain laptops with SEP 12.1 or above on them, and I tracked the issue down to being a configuration issue on the network card.
The laptops we had with the problem with were Lenovo T410, X201 and T510 and they all have the same "Intel 82577LM" network card.
The root cause was found to be that the default "Recieve buffers" setting for the network card was 80, compared to all other intel chipset witch are set to 256. So by setting the Recieve buffers to 256 and also "Transmit buffers" to 256 the problem was solved.
We also discovered that uninstalling NTP seemed to solve the issue, but it seems lige NTP is not the cause of the problem but having it installed compounds the problem so it appears more frequent.
The problem was reproducable on our systems when you tried to laucnh a java applet from an internal website where it had to download the java application, then it would just die in the download process and the network stop working.
So check the setting of the "Send Buffers" & "Transmit Buffers" in the advanced settings for your network card.