nvidia driver and flickering

[Gregory Seidman <gss+debian@cs.brown.edu>]

I'm not sure exactly what the problem is, but I am experiencing some
flickering with the NVidia-provided kernel and GLX drivers. Here's the
configuraton:

 - Dell Dimension 4500
 - Two NVidia GeForce 4 MX 420 cards, one AGP and one PCI
 - two identical Samsung SyncMaster 1200NF monitors
 - Debian 3.0 unstable, kernel 2.4.18-bf2.4
 - NVidia drivers 1.0-3123
 - XFree86 4.2.1
 - both screens running at 1600x1200x24 @ 85Mhz
 - no dotclocks/modelines defined in XF86Config

The problem is that horizontal lines, one pixel in width, between 1/4 inch
and 1.5 inches in length, are flickering on the screen. This only occurs on
the screen being driven by the AGP card. They appear to be partial
scanlines displaced to the right, since when the screen has the same color
all the way across any given scanline the problem goes away. The color of
the lines is always that of some part of the scanline to the left. The
worst situation occurs when the screen in question has one color on the
left half and another on the right.

Due diligence:

 - yes, the driver is set to nvidia in the XF86Config-4 file for both cards
 - I have switch the cables and the problem occurs on whichever monitor is
   connected to the AGP card
 - the problem does not occur with XFree86's nv driver, but I can't just
   use that since I am doing work in OpenGL

And the usual irritation: this problem does not occur under Windows.

Any thoughts on things to try to fix the problem? Is this a symptom of my
card going bad? Has anyone else experienced this with a similar
configuration?

--Greg