I agree with Toys if replacing the rad fixed the problem one of 2 things happened. The new fluid with fresh cleaning agents flushed the blockage which you had or there was a major blockage in the rad cooler which is pretty much unheard of . I would lean to the new fluid cleaning it not the rad.
On this thread it would be a speed sensor issue which can reek havoc. Fix it before figuring out your next issue if there is any after.