A couple of things could be happening to cause the depth of the dado to vary. First, the bit could be slipping in the collet, causing the bit to rise during the cut. This is an official "Bad Thing". Make sure both the bit and the inside surfaces of the collet are clean (naptha is good for this), and make sure you are positioning the bit in the collet properly - a 1/16" or so from the bottom. If the collet is riding on the little curve at the top of the shaft, you won't get a tight fit.
Or, you might have waste material blocking the bit, pushing it out of the cut. Sometimes, a spiral bit produces a better cut. Plus, for deeper dados, you may need to make multiple passes.
Feed rate is one of those intuitive things that one just learns. Too slow, and you'll likely get "burning" in the cut. Too fast, and both the wood and the router will "complain".