when i bought my Viking i was told at the Dealers shop to not use Coats and Clark thread- the machines do not like it-
so---my advice is change thread.
i have found my viking to be quite picky when it comes to threads-
some work wonderfully- others are nothing but frustration-
when ever i get a new thread- when i start having breakage problems- and i know i've changed the needle recently the next step is to try a different thread...i was very upset one time when i had spent $18 on a thread- only to get it home and find that machine will not allow it! so it's not just C&C threads it doesn't like.
but C&C threads are definite frustration every time...(not all C&C thread-only the basic sewing threads & dual duty are the ones my Viking will not tolerate...so, try a different thread- i bet your troubles disappear