I have a viking 870 with the same problem...Feed dogs would go up and down but not back and forth..If I turned off machine for a few min. it would work again until finally it worked only a few min., ea time. The repairman said it was probably the feed dog motor. Who knew there was a seperate motor for that. Later on I heard another sewer had the same problem...They still have my machine so don't know what they have done to fix..will have to wait and see. The store where I bought the machine gave me a new machine to use until mine is fixed. How wonderful is that!