I have reproduced this in 1.4.2... villagers and stairs seem to be a great trigger. Have not found anything unusual about the topography when monsters fall through the ceiling tho... but only those that jump (spiders, slimes) glitch through the floor above.
I've discovered that going 100% vanilla fixes the problem... I think there's something wrong with how people make texture packs.... as trying several different packs reproduced the problem, but vanilla always works fine. Talking version 1.4.2
Yes, I have noticed a "freeze" when any sound file initiates. Turning off the music helps immensely with that aspect of our problem. I hope this info helps a little, trying to think of everything related to this.
(BTW... I have given java up to 12gb of RAM... makes absolutely no difference)
I have this issue too... however for some unknown reason, my system recovers from the issue before the CPU usage hits 100%... on the other hand, the "lag" spikes come quite often.
I found it, found the workaround. Thanks for that.
Now, having said that:
Your search function is totally useless. It's always useless, and always will be... please stop telling people to use it first.
I searched several different ways to find "Launcher problem, bug, fix, etc" and NOTHING came up. That's why I made a new report.