XPS L502X only running at 800 MHz

Hi,

I am trying to revive an old XPS L502X, and have an issue with it running quite slowly. CPU-Z shows the CPU only running at a bit below 800 MHz, even if I run a CPU benchmark.The fun is going on and off during that benchmark, but not running constantly.

The computer is running a freshly installed Windows 10, and is in its original condition except a non-original replacement battery and possibly a non-original power supply (at least it does not say Dell on it).

Any suggestions on how I could approach/identify the problem would be highly appreciated!

Regards, Robert