This is going to be a DUMB question - and I haven't read the whole thread, sorry - but...
Do you have your machines cleaned and oiled after they've been in storage for a while, or do you do this yourself?
I ask, because I have home sewing machines AND industrials - the industrials need to be oiled and maintained regularly, and regular maintenance doesn't hurt home sewing machines, either...
Never use that blasted "3-in-1" oil on sewing machines - it tends to become a gooey, sticky brown substance later on that can really muck up your machines. I don't think sewing and fabric stores even sell the gunk anymore; but if they do - avoid it and get oil made specifically for sewing machines.
Also, check your motor belts - and NEVER over-oil the electric motor!! Over-oiling it can cause it to seize up right in the middle of a project...