(Disclaimer: I do not know your machine)
Are you sure that model number is correct? Bernina does not seem to
list it:
https://www.bernina.com/en-US/Support-US/
If you have the user's manual (absolutely necessary, IMHO)
re-thread the needle path FROM SCRATCH. Do not assume you have all
the hooks and loops threaded, do it again. You wrote "I note the
upper thread from lifting arm to needle goes slack when the lifting
arm goes down." That sounds to me like a problem with the little
take-up spring, usually the next thing after the tension disks. (But
again, I don't know your machine, I'm a Singer sewist.)
Install a NEW needle, making sure it is installed
right-way-around.
Thread the bobbin and the needle with different color threads,
to be sure you know which one is causing the problem. Sew a test.
If it has not been cleaned/lubed in a long while, take it to
your nearest shop for a thorough servicing.
HTH
Beverly