Dryer Problems part 2?

Ok....So I cleaned out all the spaces where lint might be and it was still doing the same things. I took the back off of it and got out everything I could. I ended up changing my heat setting to low heat. My king size sheets and pillow cases dried in one cycle. I was so excited. So what does this mean since it will dry just fine on low heat but not regular heat. What should probably be replaced?