what causes macbook pro (early 2008) to generate high heat

Upon retiring, I turned in my new Macbook Pro to my employer and returned to using my 17" Macbook Pro (early 2008 model).

I had stopped using it several years ago because it frequently (every 30 minutes or so) dropped the Internet connection. It also ran hot; so hot that it was difficult to hold it on the underside.

I recently replaced the original hard drive with a Samsung 250 GB SSD (850 EVO) and am running Yosemite 10.10.5. It continued to drop the connection and run hot. I used Disk Utility > Repair Permissions and that cured the dropped connection... for about 2 days!

I'm now wondering if the overheating (fan runs on high almost all the time) is somehow causing the loss of Internet connection.

Any ideas or suggestions?