In conjunction with Jim's suggestion, I would highly suggest you recalculate your protein needs. Make sure you are basing the plateau-breaking technique Jim suggested on your correct number of blocks to begin with.
Although you don't give your LBM, 15 blocks sounds a bit low for a male who is doing one hour of intense exercise daily, as you described, unless you are small in stature. If you aren't consuming enough food, it can be contributing to your plateau.
Along with Jim's suggestion, you might also try to make sure you do all you can to focus on the insulin lowering aspect of the Zone. One of the most helpful things to do would be to have only favorable veggies for all your carbs until you break the plateau. Dr. Sears advised only favorable veggies to me once when he was helping me break a stubborn plateau a couple yrs ago, and it was critical to my success. Review all Zone principles (ie: meal timing, protein requirements, etc.). Reading "Mastering the Zone" (or rereading it, if you'e already read it) might be a good idea. There's a wealth of info in that book, and you may see one little thing that could make all the difference to your Zoning efforts.
There are things, which one would not expect, that can raise insulin levels a bit, and could be contributing to your plateau. You are probably already aware of these, but I'll mention them anyway: diet soda, energy/meal bars, foods containing caffeine, not consuming enough water daily (you may need 2 or 3 times the minimum recommended 8 glasses a day), artificially sweetened foods, too many fruits in your daily in your daily food intake, not adding enough monounsaturated fat to meals adn snacks.
Good luck and keep us informed of your progress. :D