Try doing a clean install. I could point you to ten or twenty threads, that I myself have helped with, that were resolved by simply ditching the Frankenstein results of the upgrade and doing a clean install.
I'm considering that as a last resort, but would prefer not to have to start over. Especially with the amount of games and other programs I have installed, it'd take me literally months to get everything back the way I had it before. If it were a constant issues I'd have probably already done it, but when I can get 2-3 weeks of full speed and no issues for 4-5 hours of lag before it just magically fixes itself somehow, I'm putting up with it for now. The computer is still at least functional in this form, and I've got too many projects I'm using it for at the moment to try and risk a do-over that could result in a bigger problem (such as a complete failure for Windows 10 to even install at all and essentially leave me with an expensive paperweight for several weeks until I can sort it).
I'll definitely keep the clean install in mind though, but unless I can get a verification from somebody who has had the EXACT same issue that doing such will have a good 90%+ chance of fixing it, I'm going to try and explore other options for now.