I have a 2000Saturn L Series,overheats when
going down rd.replaced thermostat,flushed
the cooling system,won't overheat sitting
That certainly sounds like you have a partially clogged radiator. Idling doesn't produce much engine heat but driving requires more cooling. You can use an infrared thermometer to check for cool spots in the radiator coils.
The engine produces more heat and has a greater need for coolant at higher speeds than idle, as Gary said. You probably have a defective water pump; it's not pushing coolant at the required speed while you're driving. They're simple to replace on your car and a mechanic can do this quickly.