Quote - have you up dated the system software? should be 10.5.8 

also a big thing to try is repair permissions (applications, utilities folder, Disk utility, run program, click on repair permissions at the bottom left)

just guesses here without looking at the file.

did you buy all the memory with the mac? or get some after market?  have some mis-matched?

One other thing is to check all the files attached, try deleting all the textures, see if it renders in all gray without an issue, I have run into Images (mostly jpegs) which have "extra information" attached to them from PC's that will bug out a mac because it's not supposed to be there. 

if it renders OK in all gray try opening any base texture maps as images in Photoshop or any basic image edit and do a "save as" this will clear out the virus info..

again, just guessing here but usually when that happens it's a file attached not the computer or the application.... or memory errors

Thank you very much for these ideas!

The memory was all included. And the OS is updated to 10.5.8.

But I will definitely try your permissions and textures suggestions. Thank you! I didn't think about these 2 possibilities. And I had permissions messed up for another app ones, so it is definitely a possibility.

Hopefully I'll have some time over the weekend to deal with this, so I'll post again when I get some results.