This is one of those situations that just BEGS for a packet sniffer analysis!
Get a copy of Ethereal (and the winpcap packet driver) and sniff the traffic.
Look for things like TCP RST, ICMP status messages, etc. especially from the NT system. Also check the window sizes.
Since you mention NMLI, I assume you mean Frame Relay. Get the current traffic and error statistics from SBC to see if they can shed any light on it. The 60 seconds off and on business smacks of forced limitations.
What are the respective port speeds, port rates, etc. for both sides of the connection? If you don’t have a copy of the current contract, get one from SBC, since this might be a contractual detail issue.
Are these two sites the only elements in your WAN? Or are there other things going on as well?
The more information you can get and provide will help to nail this one down.