We had this problem to. We almost got a new dishwasher. Here's the problem. Your water is hard and the Cascade packs build up.
Here's what worked for us. First use just Lemishine (available at Target) in the open cup and closed cup with an empty dishwasher. Repeat a second time.
Then fill as normal with dishes. Everytime you wash use the following. Yes, it adds up in cost, but still cheaper than a new dishwasher which will eventually have the same problem as your old one.
Use Jet Dry Rinse aid.
Use Lemishine (available at Target here in AZ) in the open cup.
Use Finish Powerball Tabs in the closed cup.
Also, it doesn't hurt to make sure your garbage disposal is empty before running the dishwasher. Good luck.