Did this just start happening?
Sounds like you have air in the system. Make sure your radiator cap is not all gunked up from cooling system sealant tab residue. That cap is a two way valve. As coolant heats up and expands, it goes into the reservoir tank. As it cools and contracts, it gets sucked back into the tank (assuming the reservoir has coolant in it. Otherwise it sucks air). If the cap is working properly, any air in the system will get purged and little at a time with each expansion cycle and replaced with coolant with each contraction cycle until there is none left.
Sounds like you have air in the system. Make sure your radiator cap is not all gunked up from cooling system sealant tab residue. That cap is a two way valve. As coolant heats up and expands, it goes into the reservoir tank. As it cools and contracts, it gets sucked back into the tank (assuming the reservoir has coolant in it. Otherwise it sucks air). If the cap is working properly, any air in the system will get purged and little at a time with each expansion cycle and replaced with coolant with each contraction cycle until there is none left.