My car is being a pain

I only started playing with ground vehicles a couple of days ago, but I'm having problems with the damn things. I'm using a very simple script - just basic VEHICLE_TYPE_CAR parameters and a fixed velocity forwards, backwards, or angular L/R when controls are pressed.

They're fine on the flat, it's just when they try to change angles there's a problem. It seems that when the motor is on, they don't fall properly. Say that I build a flat road and then angle part of it up as a ramp. The car goes forward up the ramp but the back half stays floating in the air. Eventually, in fact, it won't go any further up the slope for some reason. When I stop the motor it falls fine into a sensible position (though not fast enough for my liking) and I can drive up and down the slope as required.

The same goes for reversing down the slope - the back wheels hit the ground and I start reversing except that the car is still at the angle of the slope, doing a wheelie. In fact I can sometimes flip the car over like that.

I have a feeling it's to do with the motor. I'm just not sure why. It seems that the car won't rotate properly when the motor is on but will when it's off.

Oh, and is there any way of getting the things to be a little more "off-road"? My whacking great steam powered thing won't even go over a tiny plywood cube unless it's sloped.