FPS counters in games usually display an average across multiple frames. That makes the number actually legible if the FPS fluctuates, but if it fluctuates really hard on a frame-by-frame, it might seem inaccurate. If I have a few frames here that were outputted at 20 FPS, and a few there that were at 70 FPS, the average of those would be 45 FPS. However, you could still very much tell that the framerate was either very low or very high, which would be perceived as stutter. Your aforementioned old games probably were frame-capped to 20, while still having lots of processing headroom to spare for more intensive scenes.
Yeah, I'm also of the opinion that Fallout 4 lacks that spark that makes a good game. It's got all of these fancy environments, characters, mechanics, and graphics (for the time), yet it just doesn't seem to all come together for me. Skyrim, while being a little tedious at times, actually feels like something you would really want to play to the end and then some. Fallout 4, however, isn't much more in my eyes than a mishmash of vaguely game-adjacent concepts that barely appear to fit together.
I also love fixing things! I just saved my sibling's €800 laptop that suddenly ran like dogshit and kept shutting itself off. The thermal paste in between the CPU die and the cooling solution dried up after a few years of use, rendering it ineffective, causing the processor to overheat. After cleaning up the old crusty gunk with some rubbing alcohol and applying new paste, all of the problems disappeared.
It turned out so well, they expanded it to the rest of the website.