An investigation of the dynamic response of a circular elastic disk excited by a moving mass-spring-dashpot system is presented. Three distinct types of unstable response are observed. A stiffness instability region occurs immediately above each critical speed of the disk. Other regions of instability arise as a consequence of modal interaction. A terminal instability region exists for all load speeds above a certain limiting value. The presence of viscous dissipation in the loading system may destabilize the total system.