Weird Visual Bleeding/Flashing w/ Radeon 9600

[ZealPath]

Upgraded my GeForce 3 Ti200 to a Radeon 9600 today (latest 3.5 cat drivers), and ZDoomGL is giving me a weird visual problem now. Basically I'm seeing monsters, decals, and a few other things (torches etc) I wouldn't normally be able to see temporarily flash through walls as I get close to them, it always happens just for a second or two, sometimes multiple times, and eventually stops, but will happen again as I get closer to other monsters/powerups/animated objects, etc.

I've tried to get screenshots but everytime I take one, the "flashed in" objects flash back out and it ends up looking like a normal screenshot, so nothing I can show you there unfortunately. Oh and it's not the "gl_clip_nearclip 0.1" issue either I'm afraid, if only it were that simple.

I did notice that when I purged textures, or changed texture detail (just messing around trying stuff), all monsters etc would flash in and out as well for a sec, what it means, I wish I knew.

Any ideas would be appreciated, trying to play like this is... annoying.