Looking at your specs I would assume its the SLi between the GTX275 and the 8600gt as none of those GPU's are hybrid SLI boards and therefore the drivers you downloaded for them wont make them scale properly.
Also what PSU are you using, its possible the PSU you have might not be able to take the load of two non-identicle cards in SLI. Thus shutting down after a while. You will need at least a 450 or even a 550real power 80plus PSU.
As far as the GPU's are concerned, I know only the Nvidia 9xxx series cards are hybrid SLI. If I were you Id get rid of the 8600gt and use it for another older PC or just sell it and get another GTX275 and a new PSU(600w+) depending on what you have.