Everyone's help has been brilliant. Thank you!
I seem to have got to the bottom of the problems:
To play safe, I need to make sure I change the needle according to the fabric.
That little tension spring thing is mega sensitive and I need to use it more often and more carefully.
I need to sew at a consistent and decent speed without stopping and starting - basting or tape can help.
Hem the garments before sewing them at the seams if I want to avoid sewing on the round. That way I can take a run-up on a scrap piece of fabric.
Your help has been really appreciated. Thank you, again!