Thermal Paste

After a while of video my computer will freeze with horizontal blocks then the screen will shut off. This would be because of gpu overheating right?

I bought arctic silver 5 on ebay for about $5, I apply it fine, it works great and the cpu temp doesn't go over 40, with no freezes or shutdowns.

But after a week or so it started freezing and shutting down again.

So I checked the thermal paste and a part of it was missing.

So I reapplied it again it was good for a while. Now it's back the way it was.

I know I shouldn't have to be putting new arctic silver 5 on it every week, so what's the deal?