Mine started crashing like that as well, but not knowing the exact time it crashes and not knowing if you receive an error it is a little hard to tell why.

In the launcher I turned on the "Open output log when the games starts" -- this opens the log and provides and output space for everything that requires to be logged, including all the mods.. mine has not crashed. I did test this and turned the output logging off and the game immediately crashed again -- once I turned it back on -- "knock on wood" no crashes.

I suggest to post your crash report as Karrthus said. However, the most likely cause is that the mods aren't updated to the forge version you're trying to use them in.

well if it has crashed you should include a crash report so people can help you with it, this would be located in .minecraft/crash-reports