T440 - network slows down/fails when pc connects

[Coolmike]

I have a small network. A Small business server and 8 clients. Recently many old machines are changed to new Thinkpad T440. After 3 new T440 I also put up a 440P and it behaved a little strange, slow hacking mouse and hanging programs. I did not take to much notice, thought the user had installed som malware or something. Users were now complaining a little bit about slow internet og network programs. Suddenly internet was not working at all. A small zyxel router broke down and I set up a small Watchguard firewall which had very big problems connecting at all and users was stil not on internet.
After a lot of troubleshooting on server and firewall a pattern showed and the new T440p machine was found to be the bandit. Because of all the time spend, I just reinstalled the T440P and updatede everything I could find. Returning to the network the machine worked fine and has been fine since (3 weeks)
Now users are beginning to report internet and network problems again. They are not happy and not patient anymore. I suspect some kind of driver or something Lenovo relatet.

Anyone have any idea where to start troubleshooting?

Thank you very much for all input.

[brchan]

Maybe the malware spread to other machines, and spread back? Have you run an entire scan of the network and laptops?

[Coolmike]

Oh sorry. No Malware WAS present and no malware now.

Thanks