Ok Tilt, the trace back to tjat IP address, from the server, is not pretty.
Whne it hits the gateway between AT&T and BT, it timesout. Continuing on there is some tyep of router flap going on near the IP address you provided.
My best guess is a bad route table entry in either AT&T or in BT's router(s). Could be a combination of both. Does not appear to be a router overload, just something programmatic with the routers involved.
I would take the PingPlot you have , save it as text, then send it to BT while notifying them there is a problem with the gateway between them and AT&T.
I cannot post the trace results as I am at home and simply do not have the ability to do the remote cut and paste and MS's brain dead telnet utility will not expland to allow me to get a screen shot.