I would say that your problem is in using universal needles.
Universal needles don't mean that they will do anything or go anywhere. They were supposed to solve the problem of the housewife having to stock sharps for wovens and ball points for knits. Unfortunately, they don't sew wovens worth a hoot and they're not much better on knits.
Your vintage machines were engineered to use sharps. Your jeans needles are sharps and that's probably the reason that they're working. Whenever I hear of a problem with skipped stitches, I ask if the person is using a universal and usually, they are.
Throw away the universals and use sharps for wovens and ball points for knits.
The vintage machines don't always do well with the ball points, but they DO work better than the universals, at least with the machines I've owned.
I normally use sizes 10, 11 and 12 standard sharps for fmq on all my machines, including the 15's, 201, 301, 316, 401 and the Kenmores. I buy them for about $15 per hundred on eBay and I use them for 4-8 hours each. When they start making a popping noise or when they start skipping stitches, I change to a new needle.
http://www.ebay.com/itm/160525454265 - 100 Organ Brand - sizes 9-20
http://www.ebay.com/itm/160575951557 - 100 Orange Brand - sizes 9-18
Either will be fine, less than $15 and the seller has been reliable and fast for me in the past.
Choose the thread that you want to quilt with and then choose the needle size by the size of thread that you're using. Too large a needle will cause skipped stitches and too small a needle will cause fraying thread.
If you will pm me with the size thread you're using and your snail mail address, I will send you some Organ or Orange brand sharps in sizes that will work for you.