Hotpoint DW leaving residue

Dishwasher has worked fine for several years. A few weeks ago it started leaving water spots and residue on the dishes. Some of the aluminum items come out tarnished. There seems to be "gunk" at the back of the tub where a check valve or something similar is located. Have used the vinegar rinse and even a commercial dishwasher cleaner. Still having the same results. Any thoughts? Thanks.