CLEAN sometimes runs into an instability when working on extended sources of fairly uniform surface brightness. It produces obviously wrong `CLEAN stripes' in your image. The Fourier transform of these stripes tends to be found in UN-sampled parts of the u-v plane. This is a complicated way of saying that the visibility data do not constrain CLEAN's interpolation between the measured visibilities sufficiently well to prevent it putting in badly wrong values.
Another variant of the CLEAN algorithm which helps suppress CLEAN stripes, is the SDI CLEAN (Steer, Dewdney, and Ito 1984) (A&A 137, 159). In this variant any point in the residual image greater than some factor times the maximum residual point is taken as a CLEAN component. The factor is less than one. Thus, when the residual image has become very smooth, this avoids introducing ripples into it (which occur when subtracting the beam from just one location) which lead to the stripes. This algorithm can be quite successful. A version of this algorithm that works on Stokes Q and U combined is available as task csdi.