Hm... ruling out virus and malware is a wise decision.
I would suggest using some hardware monitor to check whether your CPU is actually running at the correct multipliers. Occasionally some users find their CPUs aren't running at the correct clockspeeds.
Easier while gaming if you use MSI Afterburner (or similar) to monitor certain values as (hopefully) it will show how the hardware acts when those frame rates drop. I suggest CPU (all core/threads), GPU and RAM usages and frequencies; CPU and GPU temperature; fps and frametimes; and page file. Also perhaps note if there are any particular background processes which may run too; some games are more sensitive to open windows than others.
Let's see if we can get any particular info which seems to indicate something wrong and proceed from there.