Troybilt won't move when put into gear?

I got a Troybilt about 5 years ago. Recently I've been having trouble with it struggling to move forward when I'm cutting. The other day I was cutting and a huge cloud of white smoke blew out of it and although it will continue to run, it won't move forward at all unless someone pushes it from behind and gets me started. It is an automatic with about 8 speeds. Any thoughts?