Unity hangs if I add breakpoints before running

If I run my game with the debugger linked but with no breakpoints, everything is fine and I can introduce breakpoints during the running. If, however, I add the breakpoints before running the game Unity will just hang and never respond. If I try and unlink the debugger it also locks up and I must force close Unity - and loose any unsaved scene changes (Please can we have an "autosave on run" option!). The only thing I think that might be really "non-standard" is that in a manager objects start fn I load in multiple scenes with LoadLevelAdditive to merge them into one big scene. Out of interest, is there any difference to using this function or loading them in with DontDestroyOnLoad? Is doing this in the Start fn ok, or should I use Awake or even Update? Thanks