Precise timing of static components

Hello everyone,

while the rest of my experiment seems to run with very precise timing (all durations are specified in number of frames), the static component that I am using as an ISI to preload imaged does seem to cause dropped or added frames quite frequently. I have specified the duration of the static component in terms of number of frames x frame duration and it should definitely be long enough to pre-load all the images (~4 seconds). How do I improve timing of a static component? Could I, for example, wait for the next flip before starting the ISI?

All help is appreciated!