How do I make sure whether it's the CPU or memory that's keeping me from hitting 3.8GHz stable?

At 3.7GHz everything's fine. But @3.8GHz when I stress only the cpu it crashes to desktop sooner or later with that error message

I tried increasing the voltages:
NB: 1.53V
PLL: 1.6-1.66V
DRAM: 2.1V

still not stable.