Hello Margaret,
Have you checked the condition of the sponge bar? If it is worn out the main bed needles are not held tightly enough against the needle bed and although the machine might knit OK for single bed work, when you try to use the ribber problems arise.
Are the needles on both beds getting damaged? Make sure the ribber is set to the correct pitch for the type of ribbing you are trying. Also check the height adjustment of the ribber. If it is too high the needles could catch on the connecting arm.
Sometimes it helps to remove the connecting arm; while it is off the machine, check that it is not damaged. Remove both carriages and check that the needle channels are clear. Then replace them, making sure they are on the rails correctly and that they move freely over the needles independently. Replace the connecting arm, making sure it is seated properly before tightening the thumb screws. Also check that the carriages are set correctly before you start to knit.
I know this might sound obvious but if you check out everything stage by stage you are more likely to find the source of the problem.
Hope this helps,
Sue, October 2010