That's odd; my D-link D-604 works just fine, and I certainly haven't manually forwarded any ports. It may have its share of other problems, but disconnecting Messenger... Na-ah.
Wouldn't the port thing seem a bit odd, anyway? His Messenger working fine for a few minutes and then all of a sudden requiring port forwards to remain connected.
To me it sounds as though your computer is losing its Internet connection altogether every once in a while, maybe due to an unstable ISP or DSL-modem or whatever.
You could connect your computer directly to the DSL-modem for a while in order to either rule out the router or confirm that it's causing trouble. Hope this helps!
/ 
