The problem: I am getting a drive train vibration at 60 to 70+ mph. I had the Hunter Road Force balance done,an alignment ( to my specs ) and double checked tire pressures but the vibration is still there. I happened to wander into BSI ( BIG Miata Spec builder ) and asked Stu about it. He told me that there is a fairly heavy harmonic balance weight on the drive shaft behind the yoke that can separate and cause the vibration. He also said they checked with Mazda engineers in Japan because they had seen this on some of their own Spec cars and on some of their track day rentals. Mazda told him it was for "customer convenience " and only helped eliminate some inherent vibration problems to begin with. BSI then removed this weight from every car they own. They are getting my car next week as I don't have the capability to pull the yoke and do the repair myself.
I have not seen anything written about this condition so I thought I'd post this up for future reference. Stay tuned for pics and a write up. If anyone else has had experience with this please chime in.