Something has corrupted the residual data that is stored in the .dat file. The fix is to regenerate it. There are a couple of options: 1) Change the number of iterations to plot to 10 and number retained to 100. Run for a few time steps and then put the values back to the defaults. 2) Write cas & dat out then reread into a new session. This will lose all but about the last 1000 residual points but should reset the residual range as well.