Was it new or refurbished? It sounds like you have tried all the obvious fixes... A new machine should not skip unless you hit something to knock the timing off. Do you remember breaking a needle or hitting a pin? Sometimes that could do it and it means a trip to the repairman. When my timing was off it was skipping stitches occasionally on straight sewing but wouldn't work well on zig zag or FMQ at all. Try some of the fancy stitches and see if they sew well.