Because the AUTO values overvolt like mad!! Seriously put your voltage settings couple notches above auto and you have nothing to worry about. The GTL values as everything else have a default setting which the BIOS should provide to you. Max values for PLL and fsb termination voltage are 1.60 and 1.38. You should never go above them atleast not 24/7 anyway.
And if you haven't run the most recent Prime95(V25.6:shrug:) atleast 8 hours per large and small FFT:s your not stable imo. I just ran all the 3 tests available alteast 9 hours a piece. I haven't checked the IntelBurntest myself but I hear it's very good at finding problems with stability very fast.
PS. If you decide to use Prime95 you should have the round off checking setting enabled otherwise it's useless to run the program.