Since it's also predominantly skipping one needle, it could be the timing needs to be adjusted so that it's lining up better with both needles.
How does the machine sew with one needle and the widest zigzag? Try it with knit and without to see if it also skips stitches that way. Knits can be a monster on some machines. Ball point needles help, as they push the threads aside so the needle can pass between them. Sharp needles may penetrate the knit thread, and cause it to hold the thread too close to the needle so it does not loop slightly and the hook can't grab it.