Parasitic effects have recently appeared to be a potential limiting factor for reliable operation of high-power high-frequency gyrotrons. Within the domain of validity of a self-consistent monomode model, the time-dependent code TWANG has been developed with one of the aims devoted to the study of such parasitic instabilities. In a preliminary phase, the modeling of non-stationary behavior in frequency tunable gyrotrons for DNP-NMR applications have been simulated. Spurious effects such as After Cavity Interaction (ACI) in agreement with experimental observations are demonstrated and preliminary studies on parasitic oscillations in gyrotron beam-ducts have been initiated. © 2011 IEEE.