House electrical problem

Half of my house keeps losing power. I checked 2 main legs before the main breaker phase to phase 240, phase to neutral 120 both, and after the main breaker did the same measurement and got the same results. So when all the breaker are off I take measurement in one leg across the main breaker mean from in main B to out (same wire feeding the house point A to point B across the main breaker)and I get 240 and the same thing with the other wire or cable, but when breakers are on I lose voltage on one leg so one would be 240v and the other 122v but everything else would be normal checked to neutral good 120 and phase to phase 240. When it happens I check by the breaker and I find 0 volt even they are on, my double pole breaker 240 v has 120 in one wire and the other 0. And when I touch the one with 240 just by itself its start arcing and I hear that noise.
So what makes this happen since I checked all the breakers.
And does one leg across the main breaker from point 1 before the main breaker to point 2 after it should has 240 v?
My question is what makes this happen?
My house is 2 year old