A person who buys and installs their own heatsink, but can't troubleshoot a temperature-driven performance problem impresses me as an odd combination.
I expect providing support to members of the "enthusiast" PC market is an interesting challenge.
Its not like manufacturer applied heat sinks are fully reliable. I have had more than one go bad in my day, even better my Samsung DLP was the worst of them because its heat sink glue failure was on visible on the screen as white dots.
This probably threw them off.
I mean, it's just thermal paste on a fan, right? That's not exactly hard, after a fairly low threshold of know-how.
If you install new fancy fuel injection nozzles in your car, and suddenly your car is shit, then swap them back out and see if the problem goes away. After you determine that it does, you can start investigating if the installation was botched or if they are broken, or whatever. You don't need to know anything about cars to do that, it's just generic troubleshooting.