I had the same problems with fireline and penns on both 8ft lamis and st croixs.The way it was explained to me, the penn spool throws to small a lop and therefore the knot. switced to a large spool Daiwa Capricorn and haven't had a problem since. Never had a problem with the 7ft st croix and the sustain.
Go figure. The size and spacing of the guides may very well be part of the problem.