92 deville no heat at idle when driving I have some,

If you have no problems with any valve or perhaps the heater blower motor then heater core could be plugged, I have back flushed some successfully by changing the flow of coolant backward and catching back flow in a separate container. Could actually see large amounts of coolant stop leak.
1st thing to check, is coolant level full.
2nd , is the blower motor actually blowing,
Traveling down the road causes ram air to push through the heater as long as the controls are set for fresh air intake.
Remember If allumaseal will not plug a leak then more professional measures must be used. Many radiator stop leaks are so effective like bars leak the stuff that looks like rabbit feed pellets that when used professional help must be found because the radiator no longer has the circulation ability to cool the engine and if a vehicle is run any at all above max operating temperatures it will normally result in costly repairs. I have personally used fine ground black pepper and had good results but it must be very fine ground no matter what is used it must be small enough to travel through the small tubes of the radiator and or heater core. If larger particles are used coolant flow will probably be restricted enough to cause either no heat in the cabin or overheating of the engine which most certainly can cause serious damage the very first time.