I think you have a bad compressor.
It takes some experience to consistently properly diagnose the differences between cap restriction, inefficient compressor, and low charge. Some companies make it a habit that anytime they find a cap tube restriction (or what they think is a cap tube restriction), they change the cap tube, compressor, and drier. This will solve the problem since they have addressed everything, including a possible short charge.
Oh, and there is a way to size a cap tube on the fly with the system open. Read the last two pages: http://www.supco.com/images/pdfs/Man...g%20Manual.pdf