[troubleshooting] My CPU getting hot and throttling, duplicate build for little brother works great.

So I've built 2 systems for me and my little brother. We both have aftermarket cooler master coolers, and I've seated them according to every online guide I can find. Sometimes I use the vertical line, sometimes I use the dot in the middle, sometimes I spread it evenly over the whole thing. No matter what I do mine seems to run much much hotter than his. His is stable OC'd at 4.0 Ghz at ~58 C according to OCCT. Mine gets to 67 C, the CPU speed starts to go down, and it stabilizes at around 80 C while the CPU speed is throttling down to about 3.6 or so. Is it possible my heatsink is BAD? I can't figure out the problem :(