Tim,
Most of the Forecast line runs pretty true to the rated line wt. I've heard nothing but good things about them.
It could be the guides if you have major misalignment problems.
I ran into the same thing trying to use Cortland 333 line on a 5wt factory rod and on a 9wt factory rod. I wouldn't use 333 line for anything except staking tomatoes personally. It just will not shoot,expecially when it gets hot!
Just my $.02.
Alex

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When the experts disagree,the ignorant are free to choose.