If I underclock my RAM my 2400G is stable at 3.9 Ghz 1.35V. But for best gaming performance I clock my RAM at 3200Mhz and this limits my CPU overclock to 3.8 GHz.
I suggest you increase clock speed slowly, test stability with prime95, if stable -> keep increasing speed, if not stable -> increase voltage slowly. If stable again, increase speed again. Always try to use lowest voltage to reach a desirable speed. Repeat until you reach a comfortable level. 80 degree is a good limit for cpu temp under load, voltage < 1.4V is recommended.