Linksys Router Issue

A while ago, I got a Linksys BEFSR41 v4.2 router to hook 2 computers to a DSL internet connection. The router connection type is PPPoE (a manual suggests to have it this for DSL). It works fine and dandy except when I enable port forwarding for any application, in which case, every minute or so, the internet connection disconnects and reconnects some 20 seconds later. If I disable the forwarding, then the problem is solved. However, I need the ports opened for some of the applications I want running.

Can anyone help me with this problem? (FYI, I don't know much about what I'm doing.)

Have you tried resetting your router?

The other stock response is to update your firmware.

I get the same problem, if your adapters in your pc's came there or are cheap/old then it is them, we get this problem with one pc here.

OR, your ISP has DSLAM that detects different packet metrics (suggesting more computers connected to the line), and kills your connection. Is using a router allowed in TOS of your ISP? Also, the "applications" might be blacklisted in TOS as well.