This is really a hard to get problem...
2 ideas:
1) maybe part of the problem is coming from the modem... Any chance you can change any setting on this side?
2) maybe using tcpdump as early as possible to sniff what is being sent to the modem?
1) it might be related to the modem, but, if I load .62 the problem goes away - so I know it's a firmware change. I have no control over the cable modem anyway.
2) in previous threads linked in the original post another user with the same issue did capture data flow and it's as I have described.
Thanks to all for the help/suggestions, *very* appreciated as I know this problem is strange/rare.