Maybe this is helpful for some people who don't have a lot of experience with autoguiding in MaximDL.
According to MaximDL manual for best results, place a bright star in the field of view of the autoguider imager, near the center of the field. Take an exposure using the Guide tab’s Expose mode; the guide star will be picked automatically, or you can click on one using the mouse.
My mistake was that I did not select a star near the center of the field. I let MaximDL do this automatically. It selected a star near the edges of the field I don't know why
The second night I notice that the guide star was elongated in shape so maybe this was the reason of the brightness fluctuation. I tried to refocus but star still elongated. Maybe bad seeing...
The third night very near the selected guide star there was another star. Maybe the other star affected the guiding.
Next time I will follow manual advice.