Two Computers dropping Internet

[jcm05]

I have two computers on a fairly new domain using transperent proxy that drop there internet connection and only way to recover is to logoff. Somtimes if you wait 10-15 minutes they will work again. I have checked the FSSO agent running on the Domain controller and it shows the users still logged on and I can ping the computers from the same server. I the computers I can ping google.com and ping its IP address just cannot get the browsers to work. During the time I can have a laptop open on the same switch logged in with the same user and it works so its not the entire domain going down. I have contacted support thought maybe they would tell me where to start looking or logs to pull but they just said we need to remote in and look at the computers having the issue but if they cannot get to the Internet then I cannot establish a remote connection. I was also thinking maybe something on these computers could be accessing a resource on another domain we have and then the userid and password could be causing issue's with the fortigate. Any one else out there have this issue or could point me in the right direction to start looking.