Problem with a new monitor

I've recently bought an LG Flatron 795FT+. I run it at 1024x768 at 100Hz and the screen is distorted. It has problems of geometry and convergence (seems that makes that only at high refresh rates). The technical service says that the problem is my videocard, that it's not good, blah, blah, blah, that they have tried the monitor with newer cards and it's ok. The card is an Asus with the original GeForce II MX chip (not 200 or 400) and 32MB SDRAM (supposed 1024x768 at 240Hz).

It's possible that the card produces image distortion, and convergence problems, or are they cheating me????