Thanks for the very informative response and suggestions! I'm awefully tempted to go check into that right NOW, but new years eve is calling (and it's freakin cold out - I have only a snowy gravel driveway to work in)! I will pull the manifold off this weekend, check for that vertical crack, and replace the gasket....hopefully that does it.

I've run a fuel injector cleaner through one tank, and I'm working through another full tank with injector cleaner right now (I also have a surging idle, checking to see if something in the throttle body's getting stuck, but I have a feeling it's a vacuum leak). Once that tank's empty, I'll try out the premium fuel idea, if the new manifold gasket doesn't cure the issue.

Thanks again, happy new years everyone!