There are two types of bacteria that are the main workers in your biological filter. The first bacteria (nitrosomas) convert ammonia to nitrite, they are appearantly well established by your posted concerns. The second bacteria (nitrobacter) convert nitrites to nitrates, until these bacterias are able to multiply in sufficient numbers you will have high nitrite readings. You will have sufficient numbers of both types of bacterias when your nitrite and ammonia levels reach 0, and since the nitrobacters are converting nitrites to nitrates you will also see a rise in nitrate levels which is normal. I hope I didn't confuse things for you.