Lexs400

it's a 90LS. Every time the car goes over a bump or a hole, even a speed hump, the rear gives away. there's a thumping sound every time regardless of which tire goes over or the bump.
I've had the rear air suspension struts on both sides replaced, recently got the upper control arms changed as well. But the problem Persists.
What other parts in the rear could contribute to this that I could look into replacing ?
thanks in advance !