I see, Ann, that you are at the end of your wits'...
There is no day without surprises with my Premiere Pro CS5.5 on iMac OSX, I think this is going to establish itself as real **** software.
After re-installing various times the program, I was able to work again for a couple of days with those (earlier mentioned) problematic films which were blocked.
I even could start a new project without problems.
Today I re-opened an older project (of last year) at its playing in the working are shows up with constant breaks in picture and sound, it sounds all hacked, this practically every second. As if it would be totally damaged.
I have tried to do various computer re-starts; now, opening this partcular film, gets even worse, it blocks completely and I have to force quit.
Other projects seem to be OK (for now??).
Can hardly beleive that this product is still considered as being for professionals. On my other platform I still have Premiere Elements installed, and it works pefectly! This is grotesque!