Troybilt Pony 42"

Your going to have to follow the linkage from the handle downwards and look for an electrical, safety switch. It'll likely have a push button that goes in and out as you move the handle. 
 
My guess is a somewhat bad connection inside the switch. 
Usually, a good dousing of Liquid Wrench (not so much WD40)
will clean up the contacts. 
 
That or replace the switch..