1999 Sieara AC problem

I have a 1999 diesal duel wheel pick up bought new in 1999, it had a new fuel injection pump installed at the dealer under warenty a few years ago and ever since when on long drives and runing the AC it will frez the evapator coil and I need to turn off the AC to let it melt to then use the AC systome again. Have the dealer over filled the systome or under filler it with R134-A?? Thanks Drew