I had to RMA mine at the beginning of the year for pretty much the same issue. It would throttle the CPU to like 400MHz in any sort of gaming workload no matter how light. I reinstalled the OS, reset the BIOS, battery storage mode, even unplugged the battery and plugged it back in but nothing fixed it. Valve sent me a brand new unit in its place. Valve's RMA is top notch. Mine was only around 6 months old at the time so 1.5 years is awesome that they're still doing it.
At least we know all the returns aren't just being scrapped now that they're selling official refurbished units. That's where all of the RMA units are going I suppose.