I'm having that experience
I'm having that experience today on a 2005 model Prius. Just like a flashlight with 1 dead D cell among 4, it sometimes comes back to life acting normal, but within 10-15 minutes it goes back to dud mode: very slow acceleration, always-on fan, and noisy revving engine. But I was able to drive on the freeway, provided I use onramp that slopes down. Plan to replace the battery