Did you uncheck “Cruise Control” in the control options?
This can happend by an unintentional button press.
Cruise control lets the vehicle stay on the speed you accelerate to. Otherwise you have to accelerate all the time because you basically let the car roll out otherwise.
Or, if your deathmobile runs on solar power, maybe the batteries are low, so they repeatedly run our of fuel and only get power everytime the panels generate some. But then you would get the message “The vehicle is too heavy for its engines” because they don’t generate enough power.
Edit.: Just checked the picture: it is cruise control.
Otherwise you would have a green number next to your speed, that shows the desired speed value.
The wiki entry
