Whirlcool, that's a gem of a machine! I agree, a sewing machine has it's own personality. Your mother obviously knew this one's very well!
Not to be insulting, but it could be something as simple as the threading that is causing the issues. I've learned over the years to retread the machine when I'm having trouble. Check that the bobbin is in the case properly and seated in the race all the way. In my years of teaching, I've seen this happen frequently, causing the issues you're speaking of.
Double check top threading too, could be improperly seated thread in the tension disk. Always thread with the pressed foot up.
I hope this helps. Todd