Whirlpool ADP7966 Dishwasher no longer washing dishes?

Hi, Our dishwasher has suddenly become rather temperamental - somedays it will work fine and others the dishes will come out dirtier than they went in. It mostly seems to be the glasses and cups that come out dirty, but occasionally the lower shelf doesn't get clean either. The best way to descirbe the dirt is to say it looks rather like little bits of shreddies stuck on everything.
It's a Whirlpook ADP 7966, which we bought second hand. We didn't get a manual with it and I've been unable to find one on the internet.
Any suggestions gratefully received.
Thanks.