Re: Knowing when a line is too light?
Is it the G Loomis Signature series IM6 by chance? If so, that MIGHT be part of your problem if your heavier rods are all fast rods like a lot of those upper weight classes tend to be.
The Sig series was not a fast rod and you may be having a hard time finding your timing with a slower rod.
Aside from that, take that rod to a fly shop and try a few different lines. If you can't find a line that makes it happy, it may be you or the stick.