it never finishes loading the mod pack before It crashes out. all 3 pc's now.By any chance, does it involve the game just suddenly freezing and then crashing for no apparent reason? That's been happening for me over the past day or so. It started about two days ago with crashing like that once in a while, but then yesterday suddenly started happening every time I tried to play and got worse to the point that it doesn't even finish loading the mod pack before crashing. I'm using the Legacy launcher as well.
For the monster pack you need to update forge to the latest version for 1.6.4 - make backups of worlds - try out the MultiMC launcher, easier to update forge on that launcher.