I've been having some of the same types of issues with my boat, ie in heavy air, we do pretty well, light, we get beat. A number of folks have mentioned my traveler/main sheet is too tight and the boom is too centered, along with some other main sail issues. I went out the other day with a fellow and got a few things figured out, going out sat in a 3 of 4 race series sat with another club member, and will see what else we can straighten out.
I've read the north along with any other sail trim book I can get ahold of, some works, others you have to play with it etc too. Not to put it on/in your shoes, but I have no issues looking in the mirror and realizing I/crew are the issue..........it could be you, and do not be afraid to ask for help if offered etc too.
She drives me boat,
I drives me dinghy!