I have heard that the 175 experiences poor overclocking, so perhaps 2.6 is your max, sadly. But still an FX-60 for half the FX-60 price. Every overclock is different. If upping the Vcore and upping the FSB slowly past 2.6 doesnt work, this could be your wall.
However, there is a slight possibility that if your ram timings are incorrect (too slow or too fast) sometimes this can cause errors in prime which are the rams fault and not the CPUs fault. So try to keep your ram as close to spec as possible while your upping the FSB. Or you could drop your CPU down, and just run a RAM OC to determine a range of stable RAM operrating points