stressmerchant
Sounds like the bobweight may have been acting as an additional balance weight as well
After giving this some more thought, I believe that this is correct with the following observations:
For low frequency oscillations, the bobweight would be relatively tightly coupled to the elevator and would act to damp such oscillations.
For higher frequency oscillations, the bobweight would not be as tightly coupled to the elevator due to deflection of the intervening structure and the likely smaller amplitude of oscillation.
Non-use of the bobweight could then result in enhancing lower frequency oscillations affecting the center of the elevator. Lower frequency resonances are the most important ones to control.
Additionally, removal of the radiators and associated structure may have affected the aircraft's basic resonant frequencies in the aft fuselage and caused a new problem.
Have a look at the following FAA Advisory Circular. This applies to Part 23 certificated aircraft, but is good guidance for anyone contemplating aircraft modifications.
http://rgl.faa.gov/Regulatory_and_Gu...C23-629-1b.pdf