Shotgun?
Really though, you have exhibited admirable patience. At this point you are going to have to determine the value of your time. There are several alternatives:
1) Since you don't have access to other parts for testing and my guess is that the problem is the mb, I would pick a reasonable but low priced uATX board from
this list. I would probably choose the GA690G since I've heard good reviews of it. If the mb turns out not to be the problem, the uATX mb is still in enough demand that it could be sold or you could build an HTPC for yourself or somebody else. One of the good things about a uATX is that the IGP can be used for testing eliminating a possibly bad GPU.
2) In the same vein as 1) you could sell your amd cpu and get a c2d cpu and mb and use your current ram. I'm not advocating this - it's just another possibility.
3) Start from ground zero with your current components and methodically reload in distinct steps making a clear, written record and testing stability thoroughly at each step. If not able to do this in one time period, also write down thoughts which might occur about possible tests and solutions so that they can be followed up on later. If there is space available, I would remove all components including the psu from the case placing them on a non-conductive (cardboard) surface. Everything should be considered a suspect including wires/cables.
4) Take it to a shop that builds pc's and let them diagnose/repair it - probably the most expensive in dollars but the least in frustration.
So that the entire topic doesn't have to be searched for your system specs, it would be nice if you put them in your tagline. If you don't want a cluttered tagline, put a link to them as several other members have. You could remove it after this problem is resolved.
Is this external hd you refer to in it's own case? Is it connected by USB or eSATA? Does it have problems on another pc?
If you don't already own one, get a USB key so that you can backup important, non-replaceable files. You might get two since these have been known to fail occasionally.