Front end shakes and ABS light and brake light is on on 1990 Mercedes-Benz 420SEL

I recently sold my 420SEL to my sister who had a mechanic replace the brakes. Now the ABS light is on and the need brakes light is on. She has replaced the ABS system, rotors and calipers and still does not have good brakes. When driving the car for about 15 minutes it begins to shake very much in the front. What could the mechanic have done wrong. This is the second time they have repaired it and it still shakes. The light was not on a no problems until after this repair.

Asked by for the 1990 Mercedes-Benz 420SEL
Does the shaking happen under braking only if so I would suspect rotor issues or a caliper not releasing,guide pins not lubricated. Are all four brake pads evenly worn? The suspension bushings need to checked for excess play or wear, tires and rims checked, and the brake system thoroughly inspected.
thanks for your response. The car seems to start shaking after 15 minutes of driving. It seems to happen even when the brakes are not applied. The car will just start vibrating in the front and increase to the point of being somewhat uncontrollable. I will take your advise to the mechanic. He has taken the braking system apart 2 times and the problem still remains. The rotors were turned and brakes replaced. I think something is happening with the calipers but just can not identify the problem. Any other advise would be appreciated. Thanks again.