Just tried in ie8. Works for me. My ie8 security zone setting for the internet at large (including tfl) is medium-high.
Is the computer/browser in question accessing the internet through a wireless connection? If so, perhaps wireless is cutting out?
, or chrome
have the same problem?
There's always the malware possibility.
Other than that, all I can suggest is grabbing wireshark
and trying to find out if IE8 is sending and receiving the right http data, but a tutorial on wireshark won't fit in the margin here.