Heater only works when driving then blows cold when stopped

Be aware water pump is naturally less efficient when low coolant level is present - it simply can not circulate coolant at lower rpms reducing coolant flow through heater core = no heat - - - at higher rpms there is naturally more water pump suction which allows coolant to sufficiently flow through heater core = normal heater output .... All the while actual engine temperature remains unaffected. This is the most likely scenario to offer without *hands-on* inspection. Good luck!