Here’s the basic tests to run, one of them will show you what the problem is
- https://memtest.org
- sysbench memory
- sysbench cpu
- stress-ng
- gpu-burn
This article will give you the details on how to use most of them: https://www.fosslinux.com/42630/best-cpu-stress-test-performance-benchmark-linux-tools.html
Be to do them in that order, you don’t want memory issues to show up looking like cpu issues, or cpu issues to look like gpu issues.