I'm going to assume that with all of the drive replacements you've been doing, that you follow the drive upgrade guide and you're therefore sure to have used the washers to set the drives off from the cradle (you said so), and that you're sure the shock mounts are working correctly.

Given the above, the only other thing I can think of is temperature. Does this player get left in the car all the time, or do you bring it indoors?