Games lagging for hours then working on max setting perfectly

When I'm playing a game on my 13" Macbook the game will get maybe 5-15 FPS for hours no matter if it's Portal 2 or Counter Strike Source. I try to restart the game and it still lags, but whenever I go and play 1-2 rounds of multiplayer it fixes itself and gets a full 60 FPS on max settings. This trick doesn't work if I just go and play with bots for some reason. It does this every single time I get on my Macbook, and it seems like whenever the fan turns on it starts increasing the framerate dramatically. Does anyone know what's going on or how to fix it?

PS: I'm running Steam on my external hard drive.

How's the external connected?

USB 2.0 with the drive running at 7200 RPM. So 480Mb/s should be fine seeing as how it has no problem getting 60 FPS about 20 minutes later. After it stops lagging I can even completely quit Steam and go back in and start playing and there's no lag.

So long as everything is up to date with your software I'd still suspect the external.

You never get 480 transfer speed.

I'd temporarily copy your steam folder onto your local drive and see if the issue persists

480 Mb/s is theoretical speed, not actual. The actual speed an vary quite a bit depending on the drive, the chipset, the Mac and so on..

My guess to the lack of lag is that when you initially start it up, it probably loading a bunch of stuff off the external drive and lag is related to the load delays. Once everything is loaded into memory, the lag goes away..

As "mrplow" suggested, copy over the necessary folders to your internal drive to play (especially games).

External drives are great for backup and other non-time critical tasks, definitely not games..

I tried putting portal 2 on my local drive but it still did the same thing. I waited for it to quit lagging this time and restarted the computer and the game, yet this time it didn't lag. I did this process again, but I waited a good 2 hours before restarting it, which made it lag again.

I don't like overriding system temperature control but as an experiment try using SMCFanControl to increase the fan speed prior to playing and see if that changes anything
eidac

Out of interest is Spotlight working ok?

I tried the fan controls and it's still doing it, and yes everything else works perfectly including spotlight.