Raid problem on a MSI K7T266 Pro2 RU

If you are good with Windows and Raid problems, please help :

This is my PC configuration : MSI KT266Pro2RU (VIA KT266A + VIA VT8233 + Raid Promise 20265R) / Athlon XP 1800+ / 2 x 256 Corsair XMS 2400 / Radeon 7200 64MB ViVo / 2 HDD IBM IC35 40 GB + 1 HDD IBM DTLA 13 GB / SB Live 1024 / Pinnacle PCTV Rave / Lite-On 40x / Pioneer A103

I used to have 2 HHD and few months ago I tried the Raid. I had trouble with it, same as I do now, but as I did not "need" Raid at the time I did not bother...

Now, since I am short of IDE ( 3 HHD + CD + DVD), I decided to use the Raid and I found some modded bios to "upgrade" the "Lite" raid on my mobo to full raid support the Promise chip is capable of.

I have encountered or rather re-encountered a stability / freezing issue, and I am hoping to get some ideas about what is going wrong.

Whenever I copy a big video file (guess any big file would have the same result) of say 700 MB, the computer freezes around mid point of the copying process. It happens with Win Me or Win XP, and CTRL-ALT-DEL won't work, I have to manually reset or switch off the PC

Having a VIA chipset and a SoundBlaster card, I took care of the PCI latency problem sometimes ago.
Thinking this problem was related I tried many settings of the PCI latency in the BIOS from 32, 64, 96 or 128 and same freezing at all settings

Also installed the "vlatency_v019" files in Win Me and XP but no effect either... and also with the BIOS latency on or off along with this "driver" ...

The very bizarre thing is that copying "little" files works OK : I copied the whole Win/system directory and it did it fine ! Also works OK with files of 400 MB !! It just the big files over 5 or 600 MB that makes it crash...

I hope you can provide some hints or ideas about how to solve this, thanks in advance !

Lionel