You could drop your bottom cover and clear and built out dust. if this is not the problem then.

Well first of all run a program that you can monitor you CPU and GPU performance, Load, Temp and Fan speed.

This will help you determine what is really going on. Once  you sure that the CPU or and GPU are actually overheating 100C is where you CPU should start Throttling your all ready to hot by then. You may well be due to repaste your CPU and GPU. You can find lots of Videos on this procedure on YouTube.

To get this information you need to log on to  Dell Support, enter your model number, and most importantly Enter your Service Tag Number. 

That will open all the hardware information on your Particular notebook, You should download and read your Service Manual there you will find out how to open your notebook and should have a section on replacing your cooling system. actual pasting will be per the video you like the best lots of different types of Thermal Paste. I would stuffy up on all of them in reference to notebooks. things that are good for Desktops or not as good for notebooks. Liqud metal types are really good but also need to be installed by an experience person as they, if use wrong can damage your notebook.

Thanks for your reply. Can you tell me the name of such a program?

hwinfo64 is a good one and you can set up to record a log of what goes on over a period of time.