Thanks for all the help with this problem. It finally began flickering and varying the brightness to such an extent as to be unusable. It's been replaced by a Samsung 17"TFT Monitor. 1280X1024 resolution. (I was looking for an excuse anyway.) No problems at all with the new monitor jumping about so it must be an internal monitor problem, the local computer shop was fairly non-commital and to be honest they had new 17" CRT monitors for £80 so it's probably not worth paying someone to have a look inside.