delay problems

I have coded an experiment using the IAT sample experiment. In this case, I am using images for stimuli. However, as you get further into the experiment, the ITI gets longer and longer. However, the ITI is supposed to be constant. I have set the experiment to both load all stimuli first or not and it does not seem to make a difference. I think it has to do with the large number of stimuli I have (around 525). Amy ideas?