Dishes still dirty after heavy wash cycle

Our dishwasher leaves the dishes dirty, even after washing on the heavy cycle. The glasses in the top rack have food and funk all washed up in them. Anything we can clean on this or something we can do to remedy this situation? Ours is a SHU3300UC/06 model, basic model which we bought in 1999 so have had 9 years of trouble free service.
Recently we noticed that there was soap left in the soap dispenser after the machine had finished all the cycles. On further inspection we noticed that the upper level glasses, cups and bowels are not consistently clean and it appears that the upper washing arm is not working properly. At the back of the upper washer arm, there is a rectangular piece that slides over the two water outlet tubes projecting out of the back of the washer which supply H20 to the upper arm. On removing the upper arm and the rectangular connector, I noticed there is an inner place that I think is supposed to work like a washer that fits tightly over the 2 water outlet tubes in the protruding from the back of the washer. I noted that the upper opening in the washer/seal is broken so a proper seal is not established. This would cause insufficient water pressure getting to the upper arm and reduce its effectiveness. Now I'm trying to find the replacement part. It appears to be something than can easily be replaced.
DScott