It sounds like your CPU is throttling down the clock speed due to heat. The problem with these original haswell chips is that the heat spreader isn't making good contact with the CPU die so it gets very hot even at stock speeds sometimes. To fix the problem you must remove the heat spreader yourself and apply your own thermal paste from the die to the heat spreader. The heat spreader is the top metal part of the CPU.
I have included a link that shows how to do it.
https://www.youtube.com/watch?v=I_rji1Nx5qM
I have to warn you before you try this... You could break your CPU if you try this.