Initially, I thought this was a Steam related problem, since I've really only played CS:GO and HL2 recently (not including other, less intensive, non-3D games, they don't seem to exhibit this problem). Now having acquired Starcraft 2 (in prep for HotS) it also has the same problem. The basic problem is the FPS dips every few seconds or so, at roughly the same interval (hence, rhythmic). I'm on 10.8.2, on a Early 2011 15" MBP with the 6750M. When I didn't have this problem (somehow it went away for one miraculous session) CS:GO was perfect, no FPS dips, a good 60 FPS (no FPS counter, but close enough) on max everything, 1080p. In SC2, the lag presents itself everywhere, except in gameplay. In the rest areas (Battlecruiser, etc) this lag is in your face, especially when you watch any prerecorded video (like cutscenes and the news bulletins on the TVs). I play in windowed mode because I suspect it improves things, a little. One final piece of information: iStat menu is telling me I'm pulling 40-45W from the wall. Hell, sometimes my MBP uses that much just browsing the web (no flash). gfxcardstatus says the 6750M is being used. QuartzDebug doesn't seem to report anything useful. Also, I don't think it's solely an FPS issue since I think game time also dilates. So, you've managed to get this far into the post: what can I do? Could I boot into safe mode and see if it makes any difference?