How to stop soup from breaking?

A cook is experiencing a problem with their lobster bisque breaking and becoming watery during service. They prepare a fresh lobster stock from shells, mirepoix, tomatoes, and garlic, blending it to a sandy consistency and straining it multiple times. This stock is then combined with a blonde roux, heavy cream, and spices. The finished bisque is held on a steam table at 160°F for service. After one to two hours on the steam table, the bisque loses its creamy texture and turns watery. The cook has tried adding more roux and using cornstarch as a thickener, but neither has prevented the breaking. They are considering thickening with rice or potatoes to increase starchiness, but are concerned about the texture becoming gritty. The cook is seeking advice and suggestions from others to resolve this issue.