s10 heater problems

ok i have a 98 s10 pick up and the heat will work fine untill you come up to a stop or just stop and have the vehicle running... when the heat stops i felt the upper radiator hose and its not hot but when the heat works it is hot... if i sit at a stop and just rev it up the heat will work again... any help would be great thanks...

This is a classic symptom of low coolant level. From the factory, your system used Dexcool coolant, orange in color. Do not mix orange and green coolants. Top off to the proper level. The factory radiator cap has some known issues, might be worthwhile replacing it.