A scroll saw I would presume as this is the scrolling forum? My first guess, if you are cutting 3/4 inch wood is that the blade is binding in the cut and preventing the saw from oscillating. Does it happen on striaght cuts as well as curves or curves only?
If it is the above then I would suggest trying one of several things - a slower feed rate, a more aggressive blade, fitting a new blade and ensuring the blower clears as much dust as possible.
Steve.