For "free will" to exist, there needs to be the concept of "I", as in me, my consciousness, my mind etc. Now once we have that concept, there are various ways of interpreting it. Is our mind and body separate? I think for free will to truly exist, the answer to this question has to be yes. If the mind and the body are not separate, then I do not think free will exists. There's a good scene in the movie "waking life" which explains it well. When we make a decision, what is the process that's going on brain? You could start from reverse, from when the signals reach your brain, to how the signals got there, to where the signals came from, to what created the signals etc etc.. and you start to look at it and you just have to think is it really my decision? If you look at the entire time line from requiring a decision to be made to the making of the decision, at what point do you fit in and make the decision? We certainly do not have any control over how our neurones and synapses work, nor do we have control over the external environmental factors. Could temperature affect decision? Would my mentality when I was too cold be significantly different to my mentality if I were too hot, to the extent my decision would be changed? It depends on the context I guess. But even if you needed to make a decision whether to take your sweater off or not, you would take it off when you felt like you became too hot. Do you choose this moment? Do you choose when you are too hot? What would ultimately drive you to take your sweater off? You're own choice? Based off what?
In summary, I don't think free will exists. Every action that me make is due to prior action of something else. Also, if this universe was replicated atom by atom, quark by quark etc, I think everything would pan out the exact same way. You might bring quantum physics into this and the random jolting of whatever it is, but I think in this parallel universe I have just described, the "random" movements would be just as random, ie, the same.