Hair Color Keeps Changing?

I dyed my hair dark brown about a year ago. I decided I didn't like it, so three months later, I dyed it med. blonde. Well, it changed back to dark brown, so I had it colored professionally, twice, and the same thing happened both times. I don't know what I should do. I'm not liking the brown anymore than I did a year ago. I'm tired of paying seventy bucks to have it colored and then it keeps going back.