2007 - weird performance

This is not normal behavior. It sounds like a problem with the hybrid vehicle ECU which is responsible for overall powertrain control.

For fun, you might try disconnecting the negative 12V battery cable where it attaches to the body for a few minutes, just to allow all of the ECUs time to power down and clear their stored memory. Then when you reconnect the battery cable, maybe the ECUs will behave normally.

 

I doubt its serious or it would throw a check engine light. So it may be engine drivability issue. Does the engine continue to run when you come to stop? I didn't see that mentioned.

First check all fluids under hood. Then, it may be a very sooty throttle body. Very common and if its never ever been cleaned its going to be bad.
Easy to check. Get a flashlight and take off the air cleaner black plastic box. 3 clips in front and one on the side.
Look down the throttle body bore with the light. If you can, reach under on the left side and manually turn the spring loaded butterfly with you hand opening the butterfly so you can see all the way down the bore to the bottom. See how dirty it is.

Search back my old posts as I have a detailed description o how to clean that & the MAF.

Good luck and plz list your location.

 

This probably has nothing to do with your car's behavior, and unlike you, i don't have any mechanical knowledge, i just drive, but by coincidence, last year i bought an older high miles Honda Civic natural gas car (year 2000, 154K). I drove it home about 50 miles from where i bought it, it seemed fine, i had had it checked out at the Honda agency before buying it. Then, i didn't drive it for about a week (i was driving my 07 Prius to work until the car pool lane stickers expired on 7/1/11, that's why i got the NGV car, it has current stickers). Then i drove the Civic to work. It was fine. Long drive, hot July day, steep hill on the commute. Seemed fine. The next day at home, Saturday, i drove it a couple of miles to Orchard Supply. When i came out of the store and turned the car on, something seemed odd. The first thing i remember or noticed was that when i was accelerating, the engine didn't seem to fully kick in at first, and then it did, and it drove fine, but then the engine would seem to cut off, and then the engine would seem to engage and the car would lurch forward, and also it was making a worrisome noise, all the way home. I was so shaken by this that i bought a newer lower miles Civic NGV and parked the old one planning to sell it ASAP, because i need a really reliable car for work, i don't just commute, i'm in the field, all over the place. But i had to find out what was wrong with the old Civic. Discussion forums led to the eventual solution--it was the catalytic converter and to my relief, was only $300 to repair (i had feared something far worse). I don't even know if the Prius has a catalytic converter, but what you described reminded me of what the Civic was doing last year.

 

I had the exact same problem. But, it was with my 2010 Prius. My theory is that the downhill trip on a hot day overheated the traction battery. Since the HV battery fan was running at full speed. I pulled over after a few miles, turned the car off and then back on. Then the car ran normally.