Sometimes when the fourth or fifth finger is weak, the other fingers will overcompensate, or the weak finger will "try too hard". That is, if your fourth finger is weak, it may have a tendency to strike early because it's trying too hard to match the sound of the other fingers. Usually the fourth finger is the culprit here. Maybe practice just dropping your fourth finger by itself on a key over and over again.
Also, doing a five-finger drop exercise goes nicely with working on scales. Put your hands on the keyboard, really on any notes you want, then concentrate on dropping each finger effortlessly on the keys. The keyword is effortlessly. Pretend that someone tied a string on each finger and is lifting it up, without any effort from you. Then that person cuts the string and the finger drops through the key using it's own weight, with absolutely no effort on your part. This is a great exercise, I still do this all the time. Go 1-2-3-4-5 and back on one hand and do the other. Speed is of no concern with this exercise, just concentrate on the effortlessness of each finger drop. I think doing this exercise consistantly will help your eveness, along with both your tone and overall muscle relaxtion while playing.