Um, sorry for posting on an old thread, but are you playing on mindcrack? I run the same problem when i play minecrack and the thing is, theres some kind of error with the combination of all the mods in minecrack. When I researched this issue it appears to be that if you dont uninstall a lot of the really fun mods like thaumcraft and others then you wont be able to reach a full hour to two hours in gameplay on mindcrack. Sorry to disappoint but I think that we will have to wait until the programmers fix this bug. As for the other launchers, i believe that if you go into finder/library/application_support and delete the entire FTB launcher file then you may be able to play on the other packs like Retro, 3rd Party, or Beta. I would highly recommend Direwolf20 to you if you are running into this issue. Once again, sorry for posting on an old thread or if the problem has already been solved. I just now registered so im not too used to the layout of this site.
~Zeldageek_1
~Zeldageek_1