Mac Terrain rendering glitches?

First off, this is an Intel iMac, ATI Radeon 2600HD, OS X 10.5.7 ... so hurray for the "world famous" (cough) ATI drivers.

Now, what happens is that the terrain is sporadically covered with what appear to be untextured polys (?) subject to a few conditions:

• This only affects Torque but it occurs with every demo and every build with 1.8.0 and 1.8.1. Other OpenGL demos, etc don't do anything like this.
• This only happens to the terrain -- every single other object, emitter, effect, and surface is perfectly fine, textured, lit, etc.
• The glitches are STATIC when strafing and/or flying but OSCILLATE very quickly when rotating the camera even a little bit.
• Resetting the camera position/orientation produces exactly the same pattern — i.e. it's repeatable and deterministic.
• The terrain IS properly rendered when the "near field" (high rez?) ground texture is applied to the close up area (like the TGEA walkthrough demo).
• It appears as though a long-duration emitter/explosion — long enough to throw light on the terrain — will cause the area lit to be properly rendered until the effect fades.

Here are a few shots:

At first I suspected a bad shader or some other corrupted file until I went through and tried all of the engine builds in all of the project folders for both of the versions (total of 9 different executables in 6 different projects). As only one or two of these share resources, it's obviously impossible for a single corrupt file to be the cause of all of this. Furthermore, I just dl'd the 1.8.1 version, unpacked it and whammo -- same exact problem. (As I said before, hurray ATI drivers...)

I have folded in the corrections for the black texturing issue and read up on the mip-map workaround. I also noticed that the console shows that the flag for both "needsExplicitGenerateMipmap" and "HD2600DepthBufferCopy" are set to 0. Again, not entirely sure they should be set or not but noticed it was there.