C2 transitions are when the CPU drops down into a lower power state to reduce heat generation and energy consumption.
Since the problem has persisted through a scratch reinstall, a hardware problem is possible. Bad drivers
, however, are also a possibility.
Check the event log for hard drive IO errors.
Check your CPU and motherboard temperatures to make sure nothing is overheating.
Install the latest version for all your hardware drivers. Install any outstanding BIOS updates for your motherboard.