Run a de-fragment program. Clean up everything. Restart it and see if that helps. If not,check for viruses,spyware etc. If you've been downloading things online to watch free shows and movies,there's probably spyware or something on there.

you need to do cleanups now and then, your hardrive needs to be freed from the unnecessary files im not sure which program to use to do this because im a pc user but all computers are the same, but its just a cleanup that needs to be dont then it should return to normal, if it doesn't your ram may be dieing or the videocard in which case that's the worst case

contact the apple support they are specialists in this stuff xD

Have you been clearing your cookies an caches?