i'm stumped

[monocus]

my drain for my tank has been working great for years(stockman);but in the last few months it hasn't been draining properly.some days(most of the time)it drains properly and i have a fall of 4" from the top of my over flow ;other days my water level is an inch above my overflow.my water level is usually 2" from the top of my tank as i use a wave box-but on the days it doesn't seem to drain,its about 1/2 " from the top.i checked my drain and it's clean with good water flow.any body know what is happening(besides full moon;though it does happen then)or how to fix the problem without adding another drain as i dont want to tear it apart(220 gal peninsula)