1995 Jeep Cherokee intermittent heavy shake

My Grand Cherokee has the whole front end shaking at times. I have good motor mounts and have kept excellent brakes on it. I can be going down the highway at 65 and it starts and feels like it is going to fall apart in the front and is hard to steer. It has happened 6 times in 2 years, but most recently twice in a row. I just had new anti-lock brakes (total package!) On front and a broken motor mount replaced -- I think the heavy shaking has caused that. It is happening more frequently -- once last week and twice this morning on expressway! No one seems to know why this is occurring. HELP!