obviously i dont know how your network is setup, but have your it ppl tried clearing the arp cache of the respective server/router/ubr(s)?...

...after re-reading your posts a couple of times to try and get a better grasp on the situaion i'm not hopeful clearing arp cache on an intermittent connectivity issue is going to clear the issue (although it is something to be done if only to cross it off of the list of possible solutions; strange things can happen w/tcp/ip). it almost seems like an ip conflict, but the duration of connectivity is too long...