Leadtek GF2 Ultra TV-out everchanging brightness

[Leo V]

This is a funny problem... the Leadtek GeForce2 Ultra TV output (S-video converted to AV output) works fine, but its brightness changes from dark to bright and back. It stays dark for several seconds, then becomes brighter in visible incremental steps, and then slowly returns back to dark in a few seconds.

This is using an Asus A7M266 and Windows 98SE with Leadtek drivers. I have tried all allowed TV modes from 640x480x256 colors to 800x600x32-bit, and the amusing brightness problem persists. Has anyone seen something like this? I'm wondering if that's a manufacturing defect.

Thanks.
Leo