Ben used to be like that too... especially between 6-8months.. Most of the time is at the early part of his sleep. That means not deep sleep yet. teething is one of the reason... but other than that could be also over-active in the daytime and too much new things.. I tried everything i could lay my hands on. Normally when the cry starts, it takes about 15-30 mins to calm him down. Nothing much we can do except coax him. Best way I used was to drive... less than 10mins he will knock out. When he grew older ( 11-12 months was better. But they will grow out of it.
If you're a buddhist, PM me..