That is quite common, an overheating inexpensive laptop that turns unusable because it's throttling constantly.
Swapping the 65W core 2 duo CPU with a 95W quad core puts quite some stress on the stock cooling system: the cpu runs almost at more than 70+C and the fan is very loud.
I found in the street 4 Optiplex 780, 1 optiplex 380 and a small tower based on a Gigabyte LG1155 I3 CPU.
This is the modding of one of the optiplex.
This one was part of the 6 computers I found in the street. Only the PSU, mainboard and CPU were salvageable. I had some acrylic glass, paint, and time to kill.
The last of the PCs I found in the street that I could revive, canibalizing parts from the two I couldn't salvage.
One of the Optiplex I found in the street.