Pajero 3.5 SWB 1999model ABS light problem

Hi, I have this problem with the ABS dash light of my 1999 Mitsubishi Pajero. When I turn the key the ABS light lights up and then turn off like it should. Sometime after I drive several miles and apply the brakes the ABS light comes on and stay on until I shut the ignition switch off. when I start up again everything is all right then after several stops the ABS light stays on again. Can you help?