Did you remove both rear springs and compare them? Since they're used there could be an irregularity with one of them. But it is unlikely.
Since you used the rubber isolator on the upper spring perch I'd recommend double checking that the top of the spring is seated properly in the isolator. I'm sure you noticed that there is a notch for the end of the coil to fit into.
Also, roughly what is the overall spring length? Some sport springs are short enough that at full droop they can fall out of place and then when the car settles back on its wheels the spring will be out of place. Something along these lines could be your problem.