Unusual long wavelength (0.5--1 AU), low-frequency (~3 ¿ 10-5 s-1) velocity oscillations have been observed by Voyager 2 at 48 AU in the outer heliosphere (Paularena et al., 1996). The velocity oscillations appear to be superimposed on weak linear velocity gradients that are typically increasing. To date, no explanation has been advanced for the origin of either the flow gradients or the oscillations. Here we show that pickup ions can excite low-frequency fluctuations and drive the flow field linearly. The model is compared directly to velocity oscillations observed by the Voyager 2 PLS instrument between 40 and 55 AU over the 6-year period 1993--1998.