wavy lines on screen problem

got a problem where someones got a new LCD monitor and plugging it into their PC, theres vertical wavy lines (distortions). most noticeable on colour pictures. really makes the PC unuseable.
I can't remember what brand the monitor is, its at the budget end though. Only supports analogue in (and the cable is built into the monitor). The PC is a dell dimension 2400.

Tests I've done to try and fix it.
- plug monitor into another computer: I plugged it into a laptop and it works fine, even plugged into the same power socket (using same cables).
- tried a load of different refresh rates and different resolutions
- switched VGA card. only has PCI slots so I put a PCI card in, still wavy.

My next idea is to take another power pack wrong, could it be an interference problem stemming from that?
previously there was a CRT monitor plugged into this PC. Is it worth plugging another LCD in?

Any help or suggestions or even vague hitches would be much appreciated!

Thanks for reading,
Dan Gent

Could be just a duff monitor, but first of all check that there are no transformers (no, not the robots in disguise kind) near the cable.

As I said in my post, the monitor plugged into another computer works fine.I connected it into a laptop, with the monitor plugged into the same power socket still, and there were no wavy lines or any forms of distortions.
should I be checking the PCs power cable though? make sure thats not near anything? though its plugged into the same "power strip" (is that the right word?) as the monitor.

My next idea was to take round another power pack and kettle lead