a good breakfast always help as does plenty of sleep.
Is the problem when youre delivering your speech more that you get nervous and overwhelmed by it? Or are you focusing too much on the fact that its an assessment and you know that youll stuff up so it sort of compunds and you get worse?
Just remember that even though the speech is an assessment, its also not the end of the world.
Is your class aware that you stutter and then sometimes stuff up? Just let them know before you start 'you guys know that i have a bit of a stutter and stuff and it gets worse when i make speeches so go easy on me ay?' make it a bit more of a joke and theyll relax and youll relax and then it could be easier to do.
Also be aware that even though you have a time limit on your speech, try and put good quality material into a short amount of time to allow for if you do make a mistake because theres no point having heaps of good points if no one can undersatnd you and you stuff up and lose track of where you are.
Have good, easy to understand palm cards and take lots of deep breaths before you start. Use your punctuation points to take another breath.
I sound like some sort of bizarre weirdo counsellor or something. Just my opinion on what could help you
good luck.