Thanks..
By 'not using FSX' I meant that there was three-year period where I was completely inactive , but have resumed with FSX. During that period there were changes to GSX. As a result I am unable to put a specific time on when the problem started to occur. I just know that it is occurring now.
FeelThere E-Jets V2. Same thing happens with the Carenado C208B-EX.
At completion of pushback, after the 'Good Engine Start', I set the parking brakes. GSX disconnects and in the process the brakes get disengaged and the aircraft leaps forwards.