I have posted many request to help on many linux / gentoo related site...
Unfortunately no one was able to help me. I finally got this scsi driver in-stability issue fixed, so in case this will be helpful to anyone I am posting this conclusion in hope it will save somebody time as it did waste more than 36 hours of my time!!!

Here is the issue:

Bus Error

Scsi hangs when given heavy access or load

Software / OS used is Gentoo but I believe this effects all Linux 2.6.xx kernels when Adaptec aacraid or 2120s 2200s etc... is being used!!!

Here is the solution:
Get this SOURCE CODE driver from Adaptec website. I used version 2372 the file name looks like this:

aacraid-1.1.5-2372.gz

put it in your kernel source scsi driver directory like:

/usr/src/linux/drivers/scsi

then extract it with:

tar -zxvf aacraid-1.1.5-2372.gz

it will extract the files to your aacraid directory.
double check that the files are indeed written using:

ls -l /usr/src/linux/drivers/scsi/aacraid

then.... recompile your kernel and make sure you included support for aacraid in your kernel.

Tada.... no more hanging due to Bus Errors....!!!

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