We investigate the spin relaxation of two-dimensional (2D) electrons in a Si/SiGe quantum well by means of electron spin resonance. Simultaneous observation of cyclotron resonance allows us to evaluate the influence of momentum scattering on spin relaxation. We identify thus a dominant contribution due to the D’yakonov-Perel mechanism which is expected to be more efficient for slow momentum-relaxation. The observed relaxation times of microseconds can be explained, however, only by an additional motional narrowing due to modulation of the spin-orbit coupling caused by the cyclotron motion. The latter is evidenced by the observed dependence of spin relaxation on the direction of applied magnetic field which changes the cyclotron frequency of the 2D electrons.