Uncontrollable front wheel bouncing

1998 Cherokee, about 100,000 miles. With no warning, the front wheels will begin to bounce uncontrollably. This happens at slow speeds, 35mph and high speeds 70mph. Smooth surfaces, it's not like running over a bump or pot hole causes this. Can go weeks without occurance, and then it will happen twice in one day. You have to pull over and stop and then it's OK. Hard to replicate the issue for a mechanic. We've had a front end alignment completed recently and they (supposedly) checked the entire front suspension. Said the bouncing came from one of the tires being over-inflated? We thought we had it fixed, but it appened again recently.