Welcome to my world <img src="smileys/smiley17.gif" border="0" align="middle">
Zooming and plasma doesn't seem to work if you use multiple colour filters for your plasma effect. They all get displaced, sometimes moving off screen, instead of layering on top of each other. Try massively increasing the size of your plasma objects and you'll see what I mean. I tried to use plasma for waterfalls but had to abandon that idea when zooming screwed up my plasma effects.
I think there is a workaround you can use that someone suggested (I'll try to find the link when I'm back on my computer, using an iOS device atm - you could try searching the forum for it) but it's not a quick solution. Might be best to either abandon the zoom function or achieve the effect you're after without plasma.
EDIT: Here you go. Hope it helps <img src="smileys/smiley1.gif" border="0" align="middle">