Toyota Repair: air conditioning in 2000 Toyota Sienna, toyota sienna, c systems

Question
I just purchased a used 2000 Toyota Sienna and the air conditioning works great (nice and cold) but only when the van is driving.  If idling at a red light or when I am waiting in the running van while my husband runs into the store, the cold air turns warm and it gets hot in the van.

Do you have any idea what it could be?

Answer
Could be a lot of different causes for this, it may be low on freon which would indicate there is a leak somewhere in the system or there could be a restriction in the system, it will take someone that knows a/c systems and can hook up pressure gauges and knows how to diagnose the pressure readings and make a diagnosis.