In this paper, we analyze stability and /spl Hscr/;/sub /spl infin// disturbance attenuation properties for linear time-invariant (LTI) systems controlled by a pre-designed dynamical output feedback controller which fails from time to time due to physical or purposeful reason. Our aim is to find conditions concerning controller failure time, under which the system's stability and /spl Hscr/;/sub /spl infin// disturbance attenuation properties are preserved to a desired level. For stability, by using a piecewise Lyapunov function, we show that if the unavailability rate of the controller is smaller than a specified constant and the average time interval between controller failures (ATBCF) is large enough, then global exponential stability of the system is guaranteed. For /spl Hscr/;/sub /spl infin// disturbance attenuation, also by using a piecewise Lyapunov function, we show that if the unavailability rate of the controller is smaller than a specified constant, then the system with an ATBCF achieves a reasonable weighted /spl Hscr/;/sub /spl infin// disturbance attenuation level, and the weighted /spl Hscr/;/sub /spl infin// disturbance attenuation approaches normal /spl Hscr/;/sub /spl infin// disturbance attenuation when the ATBCF is sufficiently large.