Xbox 360 red dots on cold start, runs fine warm. Why?

This is a model I’m using to learn more about repair. I broke a screw head on the heat sink so I’m pretty sure that is the issue, but for reassurance that I’m understanding what is happening I’d like second opinions. I warmed up the heat sink with a hair dryer thinking maybe the thermal paste needed to warm up to settle, and booted the Xbox with no issue. It started up, it played games, connected to the internet, wrote to the hard drive. Acted like new, however after letting it cool it immediately red dots. I am trying this with all the screws and parts attached except that one single screw on the heatsink that snapped. The sink makes a solid connection to the chip, so would that one single screw really be the issue, or is there something else I’m not understanding happening behind the scenes? Thank you in advance! (Note: I have also already bridged the zero-ohm resistors known to cause the false overheat. After that I intentionally took the motherboard out to clean everything with rubbing alcohol 91%, that’s when it started red dotting again)