Hi,
I am having the same problem with the same router (DSL-2750U). Any solutions would be greatly appreciated.
It is not just pop mail, but also IMAP and Exchange. Email will work fine when I first log in in the morning, but intermittently various emails will lose connection until they are all lost. The only way to fix it is to restart the macbook.
This is a brand new router, my old router still works fine and doesn't lose connection. Etisalat (the terrible telecommunications company in the UAE) who sold me the router, say it is working fine as it holds a connection on their PC (and mine), so they won't allow me to return it and give me my money back. But it won't work with all macbooks in my house - they say it must be a problem with my macs, but as I said before, no problems with my old router.
Regards, Damien.