I discovered that the water inlet manifold was cracked. Installed the new part which greatly improved the issue but was not completely resolved. Upon further inspection it looked like the arm bearing had seen better days so, since the machine is ten years old I opted to replace the rest of the assembly. Parts came in a timely manner and the repair was easy - top rack dishes are now spotless!
By the way I am a fifty something female with no prior dishwasher repair experience -
AppliancePartsPros gave me the confidence to at least try to fix it with their very helpful follow along videos. Thank you you saved me a very costly service call!