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