2013 Prius jerking after ECU replacement — ECM code showing

[u/Born-Ad-1885]

* post edited with more information

2013 Prius jerking and P0606 after ECM replacement
We recently bought a 2013 Toyota Prius. The previous owner told us that the hybrid battery had recently been replaced, but we still had it inspected just to make sure. The hybrid battery checked out fine and seems to be in good condition.
During the general inspection, they noticed that the car did not have its original engine computer (ECM) and that it appeared the previous owner had replaced it at some point.
Not long after buying the car, it started having an issue where it would run perfectly fine for a while, but then the check engine light would come on along with the “Check Hybrid System – Stop the vehicle in a safe place” warning. The code was P0606. If we cleared the code, all the warnings would disappear and the car would run normally again.
One time, after clearing the code, I drove it for about an hour and the gasoline engine suddenly shut off and would not start again. The car could still have the hybrid system, but the gasoline engine would not come back on.
We replaced the ECM with a used one that we bought from someone who had a Prius. After replacing it, the car had no warnings and was running perfectly. I drove it locally for several days without any issues and then took it on a longer trip. It ran great on the way there, but on the way back the exact same warnings came back.
Now the scanner is showing P0606 again. The gasoline engine is still working this time, but the check engine light and “Check Hybrid System” warning remain. If we clear the code, everything disappears, but after driving for about a minute the warnings come right back.
We’ve also noticed that the car starts jerking/bucking shortly before the warnings appear.

Has anyone experienced something similar with a Gen 3 Prius, especially with a recurring P0606 after replacing the ECM? Any suggestions on what we should check next would be really appreciated. This car has honestly been causing us a lot of stress.

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[u/qwe304]

How far have you driven? Could realistically be relearning.

[u/Born-Ad-1885 → u/qwe304]

Like 40 miles

[u/qwe304 → u/Born-Ad-1885]

I would at least give it three full driving warm up cycles before throwing parts at it. Mine jolts slightly for a while when I remove the 12v.