i need some help please

i have an old 1st Gen iphone which currently has 3.1.3 (7E18)firmware on it (if i recall i used a custom firmware when i last upgraded it

the phone has been flawless since i have owned it (which was a few day before they were actually officially launched in the UK) but over the last few weeks ive noticed it some times "sticks" i.e. when playing angry birds, the game will just pause for a second and then continue....this symptom seems to be getting progressively worse and not only with angry birds, other games aswell

anyway, im thinking about reflashing it again, but because its already on the latest official firmware for this gen phone, i cant really upgrde to anything else, so my question is...will i be able to backup the phone using itunes, use a custom 3.1.3 firmware and put it on over the current one

are there any other alternative solutions to "reflash this phone" back to a clean working state?

i am thinking about getting a new one, but would still like to get this one working so i could give it to my son (he seems to use it more than i do now anyway, he's the one addicted to angry birds btw, not me)

any help/advice would be appreciated, thanks