Ubuntu :: Stop Karmic Freezing Without Going Back To Ext3?

Desktop running a clean install of Karmic with ext4, /home on a separate ext3 partition. If I attempt to use Nautilus to copy a file over 1GB (that seems to be the tipping point) from Desktop to another partition or an external HDD, or if I download anything at the same time as I'm copying any file, the system freezes completely (no mouse, keyboard lights flash and keypresses don't register), and the only thing I can do is hit the reset button. I don't know enough to figure out exactly what's causing it, but I think it's ext4; this very same hardware ran Jaunty with ext3 like a champ, but when I installed Karmic, I formatted that partition to ext4, and that's when the freezing started. In the event that I don't know what I'm talking about and it's actually video drivers, I am using the Nvidia driver (185) on a GeForce 6200 because I use TwinView and don't know how to do a spanned desktop without it. I had used Nvidia drivers in Intrepid and Jaunty, though, with no problems. I have no Compiz effects enabled at all.

I'm perfectly willing to go back to ext3 (I think I remember how to make the list to "auto-install" applications, and /home is separate anyway), but if it is ext4 causing the problem, I'm still likely to run into it because one of my external HDDs is formatted ext4, and I don't really have anywhere to stick nearly 1TB of photos and video to reformat it ext3. Ultimately, if that's what it takes to fix the freezing, I'll do it, but I'd rather not. I like Ubuntu...I've used it since Warty, but this is wearing me out. I will carefully follow instructions and do nearly anything; I just want the freezing to stop.