A method of mitigating transmission delay fluctuations in a communication system having a variable data rate and block-oriented subsystems, such as interleavers. The method comprises interspersing dummy data with genuine data to form a combined data stream for the subsystems. When the data rate decreases, the rate at which dummy data is supplied increases to stabilize the delay. In one embodiment, the dummy data is discarded prior to transmission and may be reintroduced as needed on the receiving end. In another embodiment, the dummy data is transmitted and only discarded after processing at the receiving end. For this later embodiment in a multi-tone system, the dummy data can be sorted onto separate sub-carriers for transmission and the power levels for those carriers can be dropped to reduce overall power consumption and crosstalk. The concept can be used to extend seamless rate adaptation to systems with forward error correction.