Had recently read an article on Cap Tube charging. They are not as common as they once were and I haven't worked with small HVAC systems for quite some time,
"If a cap tube system is overcharged, it will back up the excess liquid into the condenser, causing high head pressures. The pressure difference between the low and high sides of the system will now increase, increasing the refrigerant flow rate to the evaporator and overfeeding the evaporator, causing low superheats. It may even flood or slug the compressor.
Again, this is why a capillary tube system must be charged with a specified amount of refrigerant."
The link to the article:
The Critical Nature Of Cap Tubes
Cap tubes when overcharged will raise the discharge pressure (less room to condense refrigerant) and increased refrigerant flow through the cap tube. Usually when discharge pressure goes up ( on a cap tube system) so does the evaporator pressure which makes them less likely to freeze but more likely to feed liquid refrigerant back to the compressor.
This will most likely sweep the oil out of the compressor and over long periods of time damage the compressor bearings.
Is it possible that you have a low airflow problem as well? Dirty filter/coil, blocked airflow, inefficient fan?