Another handy task is qvack (derivative of the AIPS task quack and so named for reasons known only to RJS) for flagging a number of correlations after the observing set-up has changed (i.e., the source changed, the pointing centre changed, or the freqency changed). For example, it might be used if some evil-minded child turned on the family microwave oven for 30 seconds every time s/he saw the antennas start to move. If you were observing at 13 cm, here is how to get rid of such data. Getting rid of the child would be the best long-term solution.
qvack also has a keyword called force. This can be used if the observing setup has not changed, but you still wish to periodically flag some data. For example, you may have separated out individual pointing centres from a mosaiced observation, but now you are left with a file with no observing change, but with a distinct ``scan'' based regularity. This keyword lets you flag some data after a certain time has elapsed.