Discone antenna problem

[PJsBuys]

I have been using a discone antenna with my sds100 for several years with good results. In the past two or three weeks the reception of signals in the 800 mc range has deteriorated to the point where I will occasionally get something like chalk screeching across a blackboard from those stations in the 800 mc range. Other stations come in as before - those in the 100 to 400 mc range. I replaced the cable from the top of the antenna to the sds100 and nothing changed. Can someone suggest where I might look for the problem?