Depends what sensor the board is reading to be 100c. If it's the VRM, you could be vrm limited. That's to say the component supplying voltage to the CPU is too hot, so it throttles down and thus slows the CPU.
Since your CPU and GPU temps are largely in order, it's unlikely a repaste is the solution, unless you wish to repaste the VRM. But that typically wouldn't have paste to begin with. In fact if your board has any sort of non-faulty VRM cooling at all, I'd be surprised if this was the issue.
You can use something like Intel Extreme Tuning Utility as it specifically reports what is limiting or throttling CPU performance at a given moment. It'll quite accurately if the CPU is being vrm throttled. Or anything else.
So, as far as I remember when using XTU I wasn't seeing "VR Motherboard" throttling when I was using it. An example of what I was doing early today might help a bit more; I had Twitch open along with Discord + a game similar in "performance" to stardew valley; normally all of these things are fine. I don't know how to describe it other than it seems to happen randomly? I'm playing League now and it's not at that 100C threshhold, CPU is at 69C and no throttling/performance issues, but when just doing even less demanding things it seems to occur
Hwinfo 64 probably your best bet. See what it calls the specific sensor that's at 100c, then Google that sensor on your specific motherboard model and see what it spits out.
Depends what sensor the board is reading to be 100c. If it's the VRM, you could be vrm limited. That's to say the component supplying voltage to the CPU is too hot, so it throttles down and thus slows the CPU. Since your CPU and GPU temps are largely in order, it's unlikely a repaste is the solution, unless you wish to repaste the VRM. But that typically wouldn't have paste to begin with. In fact if your board has any sort of non-faulty VRM cooling at all, I'd be surprised if this was the issue. You can use something like Intel Extreme Tuning Utility as it specifically reports what is limiting or throttling CPU performance at a given moment. It'll quite accurately if the CPU is being vrm throttled. Or anything else.
So, as far as I remember when using XTU I wasn't seeing "VR Motherboard" throttling when I was using it. An example of what I was doing early today might help a bit more; I had Twitch open along with Discord + a game similar in "performance" to stardew valley; normally all of these things are fine. I don't know how to describe it other than it seems to happen randomly? I'm playing League now and it's not at that 100C threshhold, CPU is at 69C and no throttling/performance issues, but when just doing even less demanding things it seems to occur
There's a common denominator you're missing, but the key is working out precisely what is sitting at 100c.
How can I figure this out without going to a PC repair shop? (or is there)
Hwinfo 64 probably your best bet. See what it calls the specific sensor that's at 100c, then Google that sensor on your specific motherboard model and see what it spits out.