[umwtnt]

Your probably correct. The expansion valve is probably freezing up due to moisture. If you have access to the proper tools, I would recover the refrigerant and pull a vacuum on the system for about 30 minutes and then recharge it. if that doesn't do the job then you will probably have to draw down the system, replace the dryer and pull another vaccum for about 15 minutes, then recharge. Sometimes it just easier to change the dryer upfront to save yourself time.