Ok, so lets try something:
adapt what you had to read:
on=if(t<1000,1,0);t=(t+1);
That will show it for 1000 frames. T never gets reset to zero and so once it's past 1000 frames (adjust as necessary!) it will just be off. The only thing to ask is: will t be reset to 0 at the beginning of the next song. And there is some doubt in my mind here... But give it a whirl.
You could do a "slide off" by adjusting the x value (you'll have to find another vis that scrolls lyrics to see what variable to use) as follows (I'm calling the x value X, but I stand open to correction as to what the real variable is called (x1???)):
X=(X-(if(t>1000,1,0)));t=(t+1);
That will save you the headache of fade out!
You can, of course adapt that to fade in:
X=(X-(if(t>1000,1,0)));X=(X+(if(t<500?,1,0)));t=(t+1);
Anyway. If these work, it'll be a miracle. I'm going on the assumption that what you've told me is correct!!
Have fun!
P.