Intermittent ABS failure

The most likely cause is intermittent connection to a sensor (or possibly an intermittently failing sensor) which is OK with the car level and the wheels in the straight ahead position, but which breaks when wheels are turned or the body tilts.

Measure the impedance of each ABS sensor coil on each wheel with all possible combinations of suspension at maximum and minimum level and steering turned fully to the left and right. If that doesn't work, you could try detaching each sensor from the wheel and gently flexing the cable.

If the cold resistance seems to be OK, attach a sound amplifier of some sort (a walkman or similar) to one of the sensors. As you drive the car, you should hear a beep whose pitch is proportional to the speed of the car. Listen for any sudden change or dropout as you turn to the left and right. Check the remaining three sensors as well. If the sensors seem to be all right, repeat the previous test with the amplifier attached to the sensor inputs of the ABS computer.

Although a failed sensor is probably the most common ABS fault, this is not the only reason the ABS light can come on. Failure (or intermittent operation) of any of the wiring to or from the ABS control unit, failure of the ABS relay (located on the right side of the steering column support bracket, accessible once you remove the lower shroud) the fuse #14 in the main board, or failure of the hydraulic control block will also put the light on. It is also possible that you have a sensor or wiring which works fine when cold, but becomes open circuit when it warms up. The sensors themselves shouldn't get particularly warm, but the wiring might.