if you just have a coolant reserve [overflow] bottle and not a recovery bottle, you are ingesting air into the coolant system, which will cause the symptoms you describe.
a recovery system will have the radiator overflow tube enter the bottom of the overflow can [or the top, but you will have a tube or hose going almost to the bottom of the can] so the expanded coolant when hot, can enter the can. when cold, the coolant retracts and is drawn back into the radiator. the tube must go into, or be at close to the bottom of the can to work. if not, hot coolant is just pushed into the overflow can, and is replaced by air when the coolant returns to it's cold state.
this system also must use a radiator cap with dual seals.