A non-linear stochastic model for the single-server machine interference problem is investigated. The service rate is a rational function of the number of failed machines, the numerator as well as the denominator of the function being linear in n. Depending on the relative values of the parameters involved, the service rate may increase or decrease with n, exhibiting the incentive or disincentive effect. The analysis is carried out by the usual diffusion approximation technique. A notable feature of this model is the threshold effect exhibited in the mean queue size.