Bernina Bernette Deco 600 , we are breaking way to many needles. Sometimes it stiitches for 5 minutes or so nice and smooth then breaks a needle. other times we can do a complete pattern without breaking a needle. Theirs no lint build up and i cannot find any cause for this. Can someone give me an idea what is causing this? I have doulbe checked and we are using the needle called for by the manufacture, it is installed correctly and the cloth is a cotton flannel.

Taking for granted the tensions are good it has to be the bobbincase or hook race. First of all know that when a needle hits the hook and breaks it leaves marks or burrs on the hook that thread can catch on and continue to cause more issues. Take the bobbincase out and run your fingernail across anywhere the thread passes over and feel for burrs. If you can't polish off the burr with very fine crocus paper (sandpaper) you may need to replace it.
Its also possible the bobbin hook race is loose or needs adjusting. There is a set screw on the left of the hookrace on a part that has a finger sticking out. Loosen that.
While the thread drawing lever (the bar that goes across and bracket ( the stopper part by the screw on the lower right)
remains parallel, adjust the bracket
horizontally so that the clearance is 0.5 mm.
Tighten the setscrew .
Hope this helps.