Heat won't give you problems in this case unless the GPU or CPU is throttling down to prevent overheating, which is very likely, especially in laptops. Actual overheating (which is what it's trying to prevent by throttling itself down) will give you problems like instability, but shouldn't effect framerate. So keep an eye on temps and clock speeds.
It's also possible that your system is speed-stepping (underclocking and undervolting to save battery). To avoid this, set your power scheme to "Always on" or "Maximum Performance" while gaming and on AC power.
Another possibility is you have limited VRAM, and your card just can't keep up with the amount of data it must store. That would explain why it would start nice and smooth, but suddenly get worse.
In any case, the x300 is an older, low end card. If it weren't for your dedicated VRAM, your card would essentially be equivalent to a Radeon Xpress 200m. So it's also possible that as the levels get more complex (graphics wise), your card starts to show it's age.
Best bet: Close unnecessary programs, disable AA and AF, and run games on the lowest resolution that you can comfortably play at.
It's also possible that your system is speed-stepping (underclocking and undervolting to save battery). To avoid this, set your power scheme to "Always on" or "Maximum Performance" while gaming and on AC power.
Another possibility is you have limited VRAM, and your card just can't keep up with the amount of data it must store. That would explain why it would start nice and smooth, but suddenly get worse.
In any case, the x300 is an older, low end card. If it weren't for your dedicated VRAM, your card would essentially be equivalent to a Radeon Xpress 200m. So it's also possible that as the levels get more complex (graphics wise), your card starts to show it's age.
Best bet: Close unnecessary programs, disable AA and AF, and run games on the lowest resolution that you can comfortably play at.