There is a safety switch for the forward speed control that is either loose or requires replacement. On page 57 in the link below, you will see the switch, item #14. I would look at the switch to see if it is loose and if so, adjust it and tighten the screws. If the switch still acts up, replace it. The switch is normally open, so for diagnostic purposes only, you can disconnect the switch and try and start the mower to verify the condition. However, this is a safety and is required to be in place prior to starting the machine for mowing. In other words, do not leave it disconnected for any other reason than diagnostics.