I suggest you try undervolting the CPU and GPU.
Usually, NV locks the voltages and clocks, so its almost impossible to undervolt the GPU without dropping the clocks... but I could be wrong.
If the GPU and CPU are throttling, that might be the problem.
But what also could be causing the problem is that you're trying to run too demanding games for your hardware.
What kind of specs do you have and which games are you running at what settings (have you tried reducing the quality of the settings from say 'ultra' to 'high' instead ? You would not really notice any difference in visual quality).