FPS drops in games / freezes, etc.

For the past one and a half years, I have been worried about various kinds of problems with the PC, but I’ll write better about the latter ... In games, for some time now (maybe something about 7 months), the FPS has begun to noticeably sag. When there was an old Seagate hard drive that was already running on its last breath, many games that had no performance problems before began to show frequent drops in fps: 30-45. Sometimes the fps drop was short-term - a second or a couple of seconds, sometimes low fps could last longer - depending on different situations on the screen. Then I changed the hard drive - in some games, the FPS stopped dropping so often and kept around 30-45 frames for a long time, but frequent short-term drops remained. FPS could drop in some game from 100 to 80 or from 60 to 50, but it was damn noticeable ... I don't know ... it looks like friezes or some kind of loading ... during such drops in fps, the game starts a slide show for a split second. All in all, it's pretty annoying to play.

My system: Intel Core 2 Quad Q8300, ECS P43T-A2 mother, Corsair vs550 power supply, FZEX: Western Digital Black 1TB 7200RPM (recently purchased and replaced the old one in the system, it is the only one I have in the system), RAM 2x2GB, AMD Radeon r9 270 2GB video card.
Operating system Windows 7 x64 SP1.
At the same time, with such a system, FPS drops happen even in Quake4 and Doom3, which, in my opinion, clearly indicates some serious problems with the PC.

What I did to solve problems: After installing the hard drive, first of all, I reinstalled the OS, checked for bad sectors on the hard drive (no), drove the video card with FurMark (no overheating or performance drop was noticed), updated the video driver (installed early ones, including, to sense no), I changed the thermal paste on the processor, stress tested it, watched the temperature in idle and in games (everything is in order - during operation, the temperature of the cores is kept around 60-68 degrees, and in idle mode, about 40 degrees.)
I tested such games, like - Quake 4, Doom 3, Fallout 3 GOTY, GTA IV and its episodes, Call of Duty: Modern Warfare 2.
PS: I noticed that in some games MSI Afterburner monitoring showed a rather low percentage of graphics card loading. This is fine? For example, in Quake 4 - 60-80%, sometimes more, in GTA IV - 15-25%, in Fallout 3 - 15-25%