not so easy to give good advice without looking at machine as the problem good be anywhere,suggest you open machine and lubricate properly and also inspect drive gears and lobe that drive and move feed dog,could also be broken feeddog attachment point as some tend to crack and then give way when material is feed across,inspect real close.listening to your saying feed dog does not move down i think it could be loose or broken or if you lucky just need lubrication,also check for spring suppose to pull it down
johan
January 2013