T.,
My now 5 yr. old would do that and I would also try to tighten it even more and that would only slow him down a bit. And then I noticed that he was actually bruising himself doing this. The next time he did it I stopped the car and sat with him to talk about it. When I asked why he did it he stated that it "hurt" his shoulders. I then talked with him, again, how important the car seat was in protecting him and how it was like a great big hug from me. I then told him that I would loosen the straps a bit ( I left the cross piece at his chest) and if he kept his straps over his shoulder I would leave it that way. I made sure that I wasn't loosening them too much. I also told him that if he got his arms out of that, I would need to tighten it again. I told him that I loved him and wanted him to be safe. I'm not going to say that he never did it again but it stopped immediately for a while and then only once in a while. I would tell him that he wouldn't be able to go with me to... (then I listed all the places he loved to go) if he didn't keep his arms under the strap.
Try to have extra time wherever you go with her right now so that you can take a few minutes to stop and take care of the situation without being rushed. Otherwise she might only do it to see that vein bulging out on your forehead.
Hope this helps,
Evelyn