Why is my Computer freezing after 30 to 35 minutes?

[Eric]

After about 30 to 35 minutes, my computer gets incredibly slow and eventually to the point where it basically freezes and takes forever to shut down. I noticed that my RAM is continually increasing. My computer is like a year and a half year old. I got 8gb of ram, 750gb of hard drive space, i-5 processor, and its a dell inspiron 15r.
So I tried all of your suggestions. I'm about to open the computer to take all the dust out which might help with the overheating but it isnt really getting too hot. I also didnt try that last suggestion just because i cant trust a post from someone with really bad grammar like that and a link that seem suspicious. (No offense if it is legit or english isnt your first languange. Just how i am). All in all, my computer has gotten a lotttttt faster but it still freezes. After about 40 minutes, the RAM usage is maxed out and i have to restart the computer to reset it. Thoughts?