Your spec easily passes recommended requirements for both those games.
This is CSGO with same spec at similar FPS but twice the minimum. Compare settings to his and note he has both vsync and AA off.
This is Fortnite with similar spec but twice the RAM and VRAM. His VRAM pretty much stays at 2GB and RAM around 6GB though, so that shouldn't matter as long as your settings are no higher than his.
If after a settings check and/or adjustment the problem persists, check for dust buildup in the CPU and GPU heatsinks. If they need cleaning, do it carefully. With 2nd hand PCs, this kind of thing should be checked. Also check +12v voltages coming out of your PSU. It should stay at a steady 12 - 12.5 when under load and never drop below 11.5. It would be helpful if you listed the brand and model of PSU.
Monitor your temps while gaming with a tool like MSI Afterburner. On the monitoring tab to set desired sensors like GPU and CPU temps, you need to highlight them then check show on screen. Ideally games like this should not exceed 65c for CPU and 75c for GPU.
A lot of processors throttle down when they reach mid 80s though, which can cause severe frame drops, so if need be, add a fan or two to your case. You can also reapply the thermal paste to the CPU and GPU, but if you're new at that kind of thing, you'll need to watch some guides on it.