I have an Exmark Lazer HP. When I'm sitting at a stand still (engine running) on level ground with both handles in the "stop" position, if I let go of the handles, they'll slowley creep forward and the machine will start moving forward....on it's own. If I don't touch anything, eventually the handles will contiue to move forward making the machine go progressively faster. Is this normal? Shouldn't the handles stay in the stop position when I put them there....even if I let go of them?