I had the same issue with mine. I changed out the radiator cap and issue solved. Levels have stayed normal and reservoir is not losing coolant anymore.
i had a leak, check the drivers side bottom of the rad (engine side). When I took mine out, it was all red/pink. No puddles where it was parked. I only noticed this when the reservoir tank would never stay full. I thought it was an HG issue (no visible leaks). Anyway, good luck