One core goes to 100% usage and activate fan on Ubuntu 18.10,
Solution 1:
Use a tool like top during the spikes to see which processes are using up your CPU and then kill them (by closing the app or with the kill CLI tool).
Or if it's a kernel issue, you would see some errors in dmesg, so at least run the command. If there's some text that's red colored or looks like an error that could be the problem.