Sounds off hand to be thermal throttling. It is important that when the netbook is used, the vents on the bottom are kept clear - using it on a flat, hard surface is best. I might suggest a thorough cleaning of the laptop, some disassembly would be required to access the heatsink/fan. You may be able to find videos on YouTube for that or you can take it to your local computer shop
On a side note, it is physically possible to swap your CPU but doing so would depend upon support for the new CPU by Gateway