Stripes? Sounds like the resolution is too high. Fire up the Mandrake graphics wizard (I'm sure there is one) and reduce the resolution. The refresh rate can also be a problem. To be safe, set the refresh rate to 60Hz and the resolution to 640x480, and then work your way up from there. First keep on bumping up the resolution, then, once you find a good one, try bumping up the refresh rate, but be careful! If you set the refresh rate too high, your screen may not like it!
It could also be a driver issue. Try using the default "SVGA" driver. If that doesn't work, try the 16-colour 640x480 "VGA" driver.
In general, ATI cards are a pain under Linux (heck, even under Windoze), because ATI sucks.