doris, when my 8mo is overtired it can take hours and several attempts to breastfeed him to sleep, he seems to find it entertaining to crawl with just his legs while latched on (which is quite entertaining actually as he is still feeding with his bum up in the air!) or just feed for 5 minutes and then arch/scream/refuse/bite me (and grin) - but then complains when you put him down to crawl, try to play with him, put him in his bouncer, even just when you hold him - and you deduce that he is tired because of how long it has been since he last slept but WHY won't he just go to sleep?
I think the problem is that if DS is tired he needs a calm unstimulating environment to wind down before bed, he can't possibly go to sleep when he's excited by cats, toys, TV, people talking, all these things. He becomes overstimulated and upset and when I take him to a calm, quiet, darkened room to put him to bed it's like he's not overstimulated any more, but because he's so wired from all these other things he doesn't want to just go to sleep calmly, he wants to crawl around and play, which can take an hour or 2 to work out of his system and in the meantime if I interact too much, he is getting more stimulation, but since he sleeps in a sidecar cot and there are no sides on our bed, I can't leave him safely to play alone, it's often easier to bring him back downstairs and let him amuse himself while trying to keep it calm and unstimulating (Which isn't always possible)
The absolute best thing I've found for overtiredness, btw, is putting them in the buggy outside - it seems to calm DS down straight away and he loves looking at the trees, cars etc and falls asleep quite quickly (you can help this along by rocking the pushchair as well!) I only do it for daytime naps though as I can't get the buggy upstairs and he's too big for a carrycot.