firstly, i'll suggest you get a 3rd pt cooler - TX-3 should be good or CM 212+.

even most ppl able to get that frequency, you should experiment slowly since its your 1st time by increasing to 3.5ghz, then monitor the temp by stressing it and increase 50mhz and keep doing so until its not stable/temp too high.

do not try it on stock cooler, i nvr use 8120 b4, but i believe a proper 3rd part cooler is needed.