Mickey2

Smaller stitches over a bumpy seam is normal, a more even feed needs to be sorted out to get correct stitch length; there are small plastic even-out tools, and some brands (my Bernina has at least) have a longer foot to deal with sewing over flat felled seams on jeans. You can use an empty needle package too. Speed should not affect stitch length, unless you use a darning foot or free motion.

Size 70/10 needles is for fine fabrics and thread has to be accordingly, so double check that the needle hole is large enough for the thread to run through it easily.

These are factors which are very basic, but even though I know them all very well, I need a reminder now and then and go through the full check list.

Needle in the correct way; clean bobbin / feed dog area; a thread might be caught some where; check tension spring in the bobbin case; check for correct bobbin type and evenly wound bobbin; check that the top tensioner releases and grips the thread as it should. Check behind the face plate too, just in case. Also check for burrs in the needle / needle hole even if it's brand new, it can happen with the best brands even if it's a rare thing.

If the old thread is very sticky and dirty it might act up, but the general advice for throwing out old thread is mostly because it can become weak as the decades go by (not always the case, but if so it will snap). The second is old cotton thread is often thicker than todays standard sew-it-all polyester; which means some machines needs to have bobbin tension adjusted and it can be a bit cumbersome. I need to use a varying range if thread thickness from lighter than sew-it-all to top-stitch and I just have to live with adjusting the bobbin tension now and then.

Stitch length can be affected if some of the parts from the stitch length lever / knob to the feed dogs are gummed up. It might need tentative oiling the next few days to have it sorted out.

I can't think of anything else at the moment.