Occasionally when I begin my takeoff roll the autothrottle will disconnect when I select TOGA. I follow the same procedure each time i.e. establish 40% N1, select TOGA. When it happens I just rearm it and continue but I would like to know what I'm doing wrong.

Look in the PMDG settings in the CDU. Under "simulation", I believe, is a setting named similar to "manual throttle override". Set it to NEVER.

 

What this does is give priority to your hardware throttle(s) if they are moved. The problem is, many throttles suffer from a slight jittering, which makes the NGX think they were moved, thus disarming the AT, when in fact they were not.