Need a little more info: CPU, MB, Temps-idle and load, Case and air flow, PS.
Use HWMonitor ( free version is fine )
http://www.cpuid.com/softwares/hwmonitor.html
Good stuff...Do you know the idle and load temps from the CPU and GPU. The HWMonitor program will tell you both highest and lowest temps for your GTX 660 and every core on your CPU, even your HD's. Do you have at least two fans on your case? One or more input and one or more to get the hot air out! Are you trying to overclock anything? Have you installed NVidia's latest Driver from their web site? Do you have an aftermarket CPU cooler?