I work on Bev-Airs everyday. Not familiar with that particular unit but sounds like a leak. Restricted condition is probably due to moisture icing up in cap tube. It's imperative to pull a deep vacuum (300-500 mics) on these systems. Break the vacuum with nitro a couple times to get there. Pull all the insulation off the line set and see if your leak is there first. I find leaks there a lot. If the copper is badly corroded, the line set should be replaced from comp to cabinet. Hope this helps.
A lot of my equipment is on or near the beach, so 15 years equals major corrosion. I forget that equipment in other climates probably fairs much better. Of course,kitchens for instance, tend to be corrosive environments no matter where you are.