rMBP running Win7: major performance drop when plugged into charger

This is a real baffler:

I have a Bootcamped Win7 setup on my rMPB 15" (first gen) for gaming. About a week ago I noticed that it was running really poorly. Essentially what happens is that I will start a game, it will run fine for about 2 minutes, and then when the fans start to kick in, the game will start to stutter like crazy. The FPS doesn't drop, the game just stutters. It's similar to how online lag feels, but I'm not online.

Well today I discovered that it only does this when the laptop's plugged in. I can literally play my game with the computer running on battery and it's fine. But within seconds of plugging in the power it begins to stutter. Within 20 seconds of unplugging it, the game runs smoothly again.

I have tried resetting SMC, updating OS, updating drivers, chaging performance settings, using Lubbos to force the fans into max speed, but nothing makes it work plugged in. Resetting the PRAM seemed to work, but it reverted back on reboot.

I really don't have a clue what this could be. I've searched on it but can't seem to find anything. Has anyone else had this problem?