there is no service that cleans these that i know of,
most guys i know just use a mild solvent or some type of fast evaporating cleaning spray.

In most cases the cleaning will buy you some more time but in the end the injector winds up being replaced anyway.

This discussion has been made on this forum several times about quality of LP gas and the oil content that is in different brands of LP Gas. I believe it has been established that the oil content in the gas builds up and can cause damage to diaphragm, seals and in this case internal injector components.
You might want to try changing LP Gas suppliers and see if the problem goes away. ;o)

AND ultimately the problem might just be the injector itself just going bad. They are designed to last a predetermined "life cycle" span and it could just be they are wearing out after that amount of time. You would think they would last indefinitely but since the technology is still somewhat new in this industry with these type systems the lifespan is still somewhat sketchy. I'm sure in time they will design a better component that is more consistent in its durability.