I used this:
To PWM EL-wire. Very cool!
Except sometimes the frequency of the PWM interferes with the AC and causes it to flash badly.
I mean it looks bad. So I skip some of the PWM values and choose which ones look the smoothest.
It looks like a Lava lamp, just as you want.
It is much easier to do this with LED's, because you wouldn't have this interference problem.