dynamic mesh, negative absolute pressures

Jason

I am setting up a 2D problem with a thin gap (~5 microns) between a moving block and a stationary wall (lubrication problem). I have set a high pressure and a low pressure on either side of the moving block. When I solve the problem for low velocities (0.1 m/s), things look reasonable. I've ramped up successfully to 0.7 m/s, but when I increase beyond that, I get negative pressures and very high velocities in the thin gap. Anybody encounter anything like this? Thanks in advance. -Jason