I've had similar experiences. Often Firefox is what's eating all the RAM, and I'm viewing it with htop. I've had cases where the freeze is indefinite and I had to hold in the power button. Couldn't change to another TTY or even ssh into the machine. When people say SWAP isn't needed, I just get mad. I've delayed these halts a bit by having some SWAP available. If I ever get slowed mouse movement, I panic and quickly check RAM usage and determine what has to be killed or restarted. I don't really understand why people pretend this doesn't happen and that unused RAM is always wasted RAM.