HOLY SMOKES!!!

This is exactly the problem I have...and this is on a freshly rebuilt 360 motor, factory crank, offset weight crank damper, B&M offset flexplate and neutrally blanced converter. Brand new driveshaft, new U-joints, new Yokes (trans and rear end, that's billet piece). The motor was balanced at a machine shop.

I ruled out all drivetrain motion culprits since the 'wave' goes in Neutral/Park as the car sits and the RPMs are raised.

Best I can tell is that the shop either completely screwed up the balance job, or the converter itself is off-balance. The unknown in my case is whether having the converer sitting in a box for several years (like 10 actually...lol) while I worked on the car has caused some kind of damage to it.