Probably wouldn't hurt to get aboard that's closer to your weight specs. But I have an '11/'12 Proto CT 157, and I was riding it at about the same weight you are this last season. (with the blunted tips shortening the overall length, the effective edge on that board means it still rides more like a 159!)
This was my 3rd full season riding and I was getting a LOT
more comfortable with some real speed! (40+ mph cruising!)
My Proto being my "go to" board, I began to notice some of the same Squirrelliness
issues you were talking about. I asked a few people about it and they told me the rocker between the binders is what can cause the board to feel that way at speed! Especially on hardback n groomers! I was told it's just not a "bombing deck!"
I could be wrong about that, and if so, I will certainly get spanked for that here, (
) I mention it as just something to consider tho! When riding it in only a 2-3 inches of fresh? That squirrelly feeling pretty much went away! (At least it did for me.)
I would say maybe consider a new (longer) deck for you if you want a more controlled,, locked in feeling for bombing the groomers, but I don't think you need to bail on the Proto altogether! Nothing wrong with having a quiver of boards!
Most of us do! I've got three and want at least two more for varying riding conditions! (...but then I think I'm a bit of a "gear whore!") Haha!