10 minute mains on a 4wd SC is asking for problems. You guys are going to bounce off of the LVC a lot and worse off you're taking the packs all the way down on their capacity which will shorten their life over all.
The quick and easy way to test this is use the Rx8's data logging feature. Clear the log out for a fresh clean run, run the car until it stops as you mention. Review the data log. If your voltage is bouncing off of 6v and or lower, you're out of run time.