We analyze the effect of interference on traffic performance in a fixed wireless access system that uses frequency hopping (FH). The system consists of an access device that buffers traffic for transmission over N FH channels. The N FH channels are subject to random interference, hence characterized by different bit error rates (BERs). We show that, compared to a purely random FH, an adaptive FH that adjusts its hopping probabilities according to the interference condition of the target channel, can improve significantly the performance of the system, when this latter is subject to random interference. In addition, we develop an approximation methodology to model FH when interference is uniform or non-uniform throughout the N channels. By comparison to an exact analysis, the approximate models are shown to be quite accurate in evaluating system performance.