wanderin - mine fusses usually halfway through or towards the end, so same thing. I've figured out sometimes it's burps, this morning it was because I didn't notice the neck of his sleep sack had gotten wet with spit up and he didn't like that, and just Saturday, I'm pretty sure it was because he wanted to continue to nurse but didn't want any more milk (if that makes sense? non-nutritive sucking is what my LC would call it) so I offered a pacifier instead and handed him to dad and he calmed right down. weird. It doesn't seem to be the same two times in a row, but I'm glad to hear from others that I'm not alone and have some ideas of things to look for (switch sides, bounce for burps/gas, etc).
Sherlock - Are you tracking wet/dirty diapers? If he's having plenty, then I probably wouldn't worry much myself if the only problem is slow weight gain, but I might still check with a lactation consultant or the pediatrician because I like to check those kind of things.