Basically your plants need light, CO2, Macros (N,P,K), and Micros to be able to grow. If you're short on something they can't make use of the rest.
Since you've got plenty of Light, the next factor to look at is CO2. Actually you may or may not have 30ppm of CO2 in the tank. There's so room for error because of the accuracy of the tests that we use. I'd go ahead and bump it slightly just to ensure that you have plenty.
If CO2 can be shown to be a non issue, the next thing to look at is the Macros. Since both Nitrates and Phosphates are running high, it points to possibly that the Potassium isn't high enough. With the amount that you're currently dosing this is unlikely to be the problem.
When everything else is taken care of you look at the Micros, however they're unlikely to be the cause of your problems. As a result this leads us to look at CO2 and/or faulty test kits, or as you originally suggested overfeeding.
I'd cut back on your feedings as you've already stated you're going to do. Give this a few weeks and see how things go. If the Nitrates and Phosphates are still through the roof, then try bumping your CO2 levels some and give a few more weeks. And so on.
One final factor that could be affecting things is the plants that you are keeping. Some plants are nutrient sponges, while others don't tend to eat through the nutrients very fast. If you haven't got any plants that would be considered nutrient sponges then you might want to consider picking up some at least temporarily.