Well, seems its a bit of both - I brought the stb from downstairs(2000) up to try, and the problem wasn't showing its ugly head, i took the 4000 downstairs, and the video we had downstairs didn't cause the same problems...
I'm gonna ask the engineer if I can have a 2000 STB, cos its got the added bonus of having the stereo outputs on the back
If it wasn't for the fact that this 4000 box is generally slow, unresponsive, and just plain problematic, i'd just swap 'em over, and have the BB config file swapped over.