2001 Toyota Celica heater problem

2001 Toyota Celica GT
84,000 miles
not sure of engine size

My daughter's car has been having heating problem. Sometimes it works and sometimes it doesn't. It will throw out heat while she is moving but when she is idling for any amount of time, it will run cold. We have changed the thermostat and was told that the bottom hose was staying cold. She has never overheated. She hasn't changed her serpentine belt. Could this be slipping at times and causing the problem.