First, you need to analyze what the crystals are. Second, you need to have your solution analyzed for all constituents, not just what you think is in it. It is often the minor components that the supplier doesn't tell you about that cause all the problems. Check the temperature you operate at, get a hold of a phase diagram for the system and see where crystallization takes place and at what concentrations. If your piping isn't insulated and/or traced, perhaps it is time you do so.