emacs locks up after $('

[Dave Cohen <google@dave-cohen.com>]

I'm a longtime emacs user, new to this group, and looking for help.

I'm editing a file in "JavaScript Moz" mode, and I type, in the middle
of a long file of javascript code:
$('

followed by some characters (depending on how quickly I type them).
Then emacs stops accepting input, and hogs all the system CPU.  Until
I have to kill the process, losing whatever I was in the middle of.
Very frustrating.

What is this sequence of characters making emacs do, and how do I
prevent this or recover from it?

Thanks for any help.

-Dave