What you say makes a lot of sense. I suspect it was malware, but it was connected with W10 somehow because it happened only after I installed it and stopped after I went to W8.
If you had malware present with Windows 10, then it is possible it was wiped and removed by installing 8.
Exactly. Time to try W10 again and be careful about what software you install.
Have Malwarebytes or similar handy never hurt anyone.