OK, just need to clarify what you mean when you said this:
BTW the flex disks are in good condition and the RPM's are at 6000.
WHEN are the rpm's at 6000?
What I thought you meant is that the rpm's are at 600 (not 6000) when the thumping occurs. Alot more than thumping is going to occur if you shift from N to D at 6000 rpm!