If it is 1.6 gzh idle then it is probably just the intel speed step. My 2500k idles at 1.6 and then under load goes up to 4.5 ghz. If that isnt it I would try clearing the cmos see if that does you any good. Try playing a demanding game or running a benchmark or something to see if the frequency steps up while under load. Alternatively you could go in the bios to see if speed step is enabled.