see the replay.

i never seen this behaviour before i started using the lfs dashboard outgauge application, so i'm not sure if outgauge is to blame.

flip a car over, rev it up to whatever, put it in neutral, turn the engine off, and play with buttons (flash, handbrake, gas pedal)... the speedo goes from max to ~100kmh, and then bounces between 0 and ~100.

replay attached, skip to about half-way through.

I can confirm, but I'm sure that it can happen without any OutGauge applications running.

I didnt watch the replay, but I think you are talking about the car auto staling by not moving in a time.
If the tyres where rolling when that happens, when you press any button the tyres get the speed it had when the "physics sleep" happened. It's meant to work that way