Hey,
clock speeds adjust automatically according to what your card is being used for at the moment. If it's just in idle state then it will lower itself to preserve power and reduce heat. If you start playing some 3d intensive game the clock speed goes up so the card can work at its max capacity.
If the clock speeds fluctuate a lot during game play then you are probably faced with "throttling" due to high temperatures. If the temperatures increase to a dangerous levels the card would reduce its speeds to try and reduce the heat - lower speeds - lower temps.
If this is the case then you should look into solving that heat problem as it not only gimps the GPU productivity but it may shorten its life span.
Open your PC case and take a look inside. How is your cable management? Do the cables look like spaghetti blocking possible airflow? If so try to tidy them.
Does your PC case get enough ventilation - do you have any case fans -a good balance of intake fans - blowing air in and exhaust fans - sucking the air out is important.
Also check if the GPU fan is spinning properly and isn't making some funny rattling noise. You can use compressed air can to blow the dust out. If this doesn't help you might need to remove the GPU cooler, clean the thermal paste and reply a fresh one.
There are quite a few things that could be done