AJM: Severe Load Pressure Oscillations

Hi!

Today, while driving on the highway, my 1.9L PD 115 hp AJM engine suddenly went into limp mode. I took the next exit, turned off the engine, and then restarted it.
Initially, the driving felt normal, except for the fact that my LDA (likely referring to a pressure gauge) was spinning in a circle. Even with moderate acceleration, the needle exceeded the 1.5 bar mark. When I pressed the pedal harder, the needle managed to return to 0 bar - clockwise, as it were.
After about an hour at the destination, I got back in the car and drove off – everything was back to normal.
Slight overpressure up to approximately 1.3 bar.
Back home, I immediately plugged in the laptop and read the error codes.
It didn't reveal anything more than that the pressure limit had been exceeded.

What could be the cause of such excessive over-revving? The engine doesn't have a chip, only a box, but that box wasn't connected.