After engine is warmed up let it idle and pop the hood.
Turn heat control to HOT
Feel both heater hoses at the fire wall.
If both are HOT then heat control valve/by-pass valve is working.
One might be slightly cooler but only slightly, if it is only warm then valve is not opening all the way, or heater core is plugged up.
If both hoses are HOT then it's a Blend Door problem.
A/C will cycle on/off in defrost for sure, it is used to dry the air blowing on windshield.
It might do that in Heat only for the same reason but not sure on a '98.