Cryopump removes gas molecules by condensation and adsorption. Therefore, cryo-surface temperature and corresponding vapor pressure influence directly the pumping performance. If the surface temperature of any part is neither low nor high, there occurs the desorption of gas molecules condensed or adsorbed, and the emitted molecules can be captured again, which leads to a time-consuming and fluctuating change of the pressure. Though every gas can show such a pressure instability at a specified temperature range, the instability generated in a sputter system using Ar as a working gas and operating with a cryopump is especially undesirable. In this paper the cause of the argon instability is analyzed and corrective is provided through the measurement of the Ar recovery time.