Strange thing happens when I bias my amp

I tried to bias (at 100 mA) my newly acquired used Cary SLI-80 integrated using a standard multimeter set to DC current. WIth no input signal the bias current was over 200 mA initially, then decreased (seemingly) exponentially to 0 with a time constant of 1 to 2 minutes. This happened in both channels. The amp works well and sounds great to me by the way. I am curious as to what's going on and would like to know if there is a problem I need to address. Any advice from members with expertise in electronics is appreciated. Thanks in advance. Dave