FLASHING RANDOM TEXT at FIXED INTERVALS, HELP!

1
Hi there,

I'm busy writing an experiment and the basis is there, but since I
use MATLAB and PsychToolbox only for a week now, I'm stuck again.

My idea is as follows:
http://www.geocities.com/jsbenjamins/personal/idea.htm

I've got the modulating background working now and I can put text in
the black spot as well, but I want the text to be random, only
showing for say 100 ms and i want this at fixed intervals. But how to
do that?

HELP!

greetz
Jeroen