I’d blame the imperfections in float precision here, they would add up over time. Could you confirm this is getting worse and worse as your micro machine spirals out of control.
Here’s a couple of alternatives:
A: Stick & string - attach the static mesh to the business end of a springarm.
Rotating the Springarm in place moves the car forward/backwards. Changing Springarm length serves as “steering”. Not a very flexible approach but it’s robust and simple to implement and control.
B: For that semi-realistic feel, I’d create a circular spline and add a dummy target.
Accelerating would push the dummy along the spline and the car would interp it’s location and rotation towards the dummy with a fairly high InterpSpeed. Steering is realised by resizing of the spline.
A bit more complicated to implement but quite flexible. If you are after a simplified driving model, you could probably skip the dummy altogether and just move the car’s mesh along the spline.