  The recently formulated retarded gravitation theory (RGT) explains the
non-Newtonian velocities of stars in spiral galaxies, *without any new
hypothesis*, and may hence be tested even in the laboratory. However, doubts
have been expressed that those higher rotation velocities in RGT may be due to
instabilities. We resolve these doubts by solving the full functional
differential equations of RGT for a model 2-body planetary system. The solution
is stable and closely agrees with the Newtonian solution for this planetary
case. Thus, the big difference between RGT and Newtonian gravity for a spiral
galaxy is not due to any instability in RGT.
