Sporadic slowdown in a few games.

[miseru99]

:3 Maybe try playing in window, or switch to window fast when the problem happens, and check EE/GS% at the top of the window displaying the game. This could actually say more about the problem than blindly guessing. If one of them reaches 100% during your slowdown it's actually your cpu reaching it's limits. Sure it feels unlikely, but it could happen for quite a few of reasons.

Game could have some major slowdown in such moments that slow you down, using recommended speedhacks would probably help(no need for MTVU one), couse between using them and not is often a world of difference and the recommended ones doesn't really decrease compatibility or ruin anything, never had any problems with them and decrease in cpu usage is always nice for lots of reasons even if your cpu is overkill for such simple games.
Other thing could be your processor throttling, couse some problems with your system, could be temperature, weak noname psu, bad oc, etc.
It could also be something as trivial as too agressive power saving mode in windows which switches off most of your cpu or gpu power thinking it's not needed as pcsx2 runs only on 2 cores and is light in those games on your system as you said. It could potentially create a lag before windows would consider pcsx2 should get more power back leaving you in low fps for a while from time to time.

[FlotsamX]

:3 Maybe try playing in window, or switch to window fast when the problem happens, and check EE/GS% at the top of the window displaying the game. This could actually say more about the problem than blindly guessing. If one of them reaches 100% during your slowdown it's actually your cpu reaching it's limits. Sure it feels unlikely, but it could happen for quite a few of reasons.

Game could have some major slowdown in such moments that slow you down, using recommended speedhacks would probably help(no need for MTVU one), couse between using them and not is often a world of difference and the recommended ones doesn't really decrease compatibility or ruin anything, never had any problems with them and decrease in cpu usage is always nice for lots of reasons even if your cpu is overkill for such simple games.
Other thing could be your processor throttling, couse some problems with your system, could be temperature, weak noname psu, bad oc, etc.
It could also be something as trivial as too agressive power saving mode in windows which switches off most of your cpu or gpu power thinking it's not needed as pcsx2 runs only on 2 cores and is light in those games on your system as you said. It could potentially create a lag before windows would consider pcsx2 should get more power back leaving you in low fps for a while from time to time.

I ran it windowed for some time. Neither GS or EU went over 80%, and those percentages were at the most hardware-intensive parts of the game.

It's not the temperature (highest it's EVER been was 62C, and it wasn't on PCSX2), PSU (It came highly recommended and I installed it over the crap one included with the other components myself), or a bad OC (it's at stock clockspeed). I did however notice that the power setting mode was on balanced, and set it to performance instead.

The problem was slightly related to the sound plugin. Using timestretch caused the audio distortion I was talking about. When disabled, audio distortion becomes infrequent-to-nonexistent. I still get infrequent skips in sound, but it's nowhere near as bad as it was.

[naoan]

Despite the warning, I always find async mode of spu2-x work best.