If it won't go above 1.6GHz even when under load then I'd suspect heatsink/fan problems as the chips contain thermal monitoring that limits their speed when the temperature gets high. [by TrevorH]
Earlier today I took the heatsink and fan off my CPU, cleaned all the dust off the heatsink/fan and replaced the thermal compound (with CoolerMaster IC Value V1). I have also re-installed Windows on my new SSD. I am running a hardware monitor tool and with 100% CPU load it says the core temps are as high as 90C.
My laptop has done a thermal shutdown several times tonight due to high temperature readings. The computer however is not running at a temperature any higher than it normally does. I think the temperature sensor is malfunctioning and reading the temperature incorrectly. It is reading upwards of 250 degrees Fahrenheit but it is only warm to the touch. There is no way it is running this hot.