My PC Build is Having Performance Issues, Any Suggestions?

I've had my PC build for a while and have upgraded the hardware here and there, but recently I've been experiencing significant stutters, jitters, and skips while playing games like CS2 and Marvel Rivals. The odd thing is that the longer I play, the lower my FPS seems to drop. I've updated all my drivers and even rolled back to a previous version to troubleshoot, but that didn't help. I even hired someone from Fiverr to do a remote check on my PC, and all the diagnostics came back fine. They suggested it might be a RAM issue, but I feel like there might be a software problem too. I ran a Heaven benchmark and scored 4415 with settings maxed out at 1440p. However, I noticed my minimum FPS was just 9 - is that normal? Any help would really be appreciated!