I definitely noticed this today in stop-and-go traffic during rush hour. I was trying to think what parameters would cause this and why the engine might respond this way:
- It was fairly hot outside (90*F) and I wasn't moving that quick (water temp showed medium like normal).
- AC might have been on (fan speed 1 or 2), can't remember.
- I was in first gear but mostly rolling downhill slightly with the clutch pressed in fully.
- Once I got up to some speed (10mph or so), I'd roll in first/second gear and engine brake back down and repeat.
So next time it happens try to note temperature, AC on/off (or fan), your gear, and recent driving (if you came from high speed, have been low-speed rolling, etc). While Toyota/Subaru figure this out on their end, we can try to solidify why it occurs, whether or not it's intended behavior.