Boromir's very first idea at the council was to use the ring, and although he heard the reasons why not, he did not really believe them. He accepted the decision of the council more because he was outvoted than because he was convinced. I don't think the idea of using the ring disappeared... And he kept his doubts close, to be sure, which worked with everyone but Galadriel.
And lets not forget, Tolkien tells us through Galadriel that Boromir will be the first, but not the only one of the fellowship to be taken by the evil. If Boromir was to be the only weak link, Frodo would not have had to leave everyone behind, only Boromir. Frodo was told that one by one it would take all of them (except for our Sam) If the lure of the Ring is as evil as Tolkien tells us it is, it would have found a way of corrupting all of them in time. Let us not forget the way the end plays out.
So Boromir was only the first to go, and his susceptability to the temptation was not inherently evil, but was formed out of his patriotism and desire to find something, anything, that would save his people. He had his father's distrust of the wisdom of wizards and elves perhaps, and temptation re-organizes our thoughts and priorities.
There's no heat... it's a discussion!
Plus, based on the conversation between Faramir, Denethor, and Gandalf in ROTK, Boromir had his marching orders from his father. To get the ring and bring it back to Minas Tirith.