Yup, your cpu is too hot and starts to throttle (lowering cpu speed).
Best thing you can do now at this point is to check if the cpu cooler is attached properly.
For now i would advise you to remove the cooler, remove any sign of thermal paste and get some new high grade thermal paste like arctic silver.
Then apply new paste on the processor (i usually do tiny dots on the corners and a slightly larger one in the middle) and re-apply the cpu cooler.
After that see what the temperatures do for you then, my bet is that it will allot cooler.
Btw, what cooler are you using atm?