Acer Aspire 7551-7422 problem...

Hello.
I'll try to explain it as good as possible.
My laptop started to froze in the middle of a work. I thought that this was a virus. I formated it. After the format, the exact same problem appeared (after few minutes of work, the laptop would froze). So I tried to format it again, just on this time, the laptop froze in the middle of a format.... From there, long story short, I replaced my hard drive, and the laptop works. However, it works approximately for 40 min. and then it freezes again.
# Side note: About a year ago my laptop started to overheat and shut itself down, so I bout a "Cooler Master" for it. It seemed like I solved the problem. Now I think that the reason it freezes is because my cooler system (the fan inside the laptop) works very poorly.
My theory is if I'll change the fan for a new one, the laptop will work fine.
The other theory is that there is something wrong with the motherboard....
Well, I'm not a big expert in the computer area... so I hope to find here an answer/ solution for my situation.

 

Check the vents are clean and free of fluff.
A possible cause of a computer freezing is overheating .