i know you said you are using "tried and tested" thread...but i would try changing to a different thread any way and see what happens-
i own 3 different machines- one of which is very picky! (being the long arm) i can buy thread and use it with my viking and do just fine- then try the same thread for quilting- go through tons of frustration (telling myself-it can't be the thread i used the thread and it was fine) then, as soon as i break down and change the thread-all the problems go away.