I imagine God not existing all the time...

So in considering the ontological argument I find the weakest point to be the fact that Descartes states that "I am not free to think of God without existence (that is, a surpremely perfect being without a supreme perfection)" (7:67). There are a few problems I find here. First off, unlike Descartes finds, I am pretty sure I can think of God as not existing. I do it all the time. I realize here that he draws on the fact that the idea of God is that of a surpremely perfect being. Furthermore, that his perfection entails existence as part of its essence, for to not exist would be to lack a perfection. However, why does perfection necessarily have to include existence. Descartes seems to just throw it out there that existing is a perfection, but does not really give any reason for it. However, even if we grant that existence is a perfection and thus to think of a surpremely perfect being, God, would be to think of him existing, what is to say that we actually do have the thought of this surpremely perfect being. I do not find it strange to argue that the perfection most attribute to God is actually beyond our grasp. To think about God being everywhere at all times, existing for eternity, omnipotent, omniscient, surpremely good, and other such ideas people label God with, seems like a pretty impossible task. We cannot begin to comprehend what eternity even means. We can state that to be perfect means to have these attributes, but at the same time we most likely do not even know some of the properties that a perfect being would have, and we could equally be mistaken about others because our intellect is not advanced enough to comprehend perfection. For us, surpreme perfection is just a term. Thus, even if we cannot think of a surpreme being without thinking it must exist, why can't we be mistaken about this? Who says we can even think of God, a supreme being? If we cannot even think of God the argument clearly fails. Thus I find Descartes' weakest point in his ontological argument to be the necessity of existence in perfection without a good argument behind it, and the assumption that we can even think of a perfect being and be correct about it.