A while back I updated to the 27x.xx set of drivers from the 266.58 drivers and LoL and Source games (Portal 2, CS: Source, TF2) appeared to be capped at 50 fps for a few minutes, then jump to 60 fps for a few minutes (all of this with v-sync on and independent of what was going on in game). Without v-sync I can hit in the hundreds of fps in all these games.
I clean installed 266.58 and was fine.
In my infinite wisdom, I tried to give the new 275.33 drivers a try. Now I get drops (once again independent of what's going on in game) down to around 45-50 fps for a second, then back to 60.
My specs (that seem relevant):
i7-920 @ 3.6ghz
System started crashing frequently, so I took out my old 470. The crashing persisted and I couldn't even boot/post for a short while. All I got was a black screen. I checked all of my RAM and cleared the CMOS, un-overclocked and now everything seems back to normal.