wirsbo 311 mixing controls

rod robbins

I have a Wirsbo 311 control with a Belimo mixing valve.When mixing, the valve opens fully and the temp climbs to around 118 then the valve fully closes and the temps fall quickly to around 80 F.at which time the valve then opens fully again.This whole up down cycle takes about a minute.I thought the control found the temp required and held the valve in that position with only minor ajustments.I would like to hear your comments.
Thanks
Rod