Captured signal slightly out of sync on different video cards

i’m having a problem with the captured/shared memory signal:
the stream will play with different delays (2-3 frames at most) on individual videocards.
no big deal… except when there is flashy/fast changing content, ie.strobe.
could it be a problem of accessing the shared mem at different times for individual cards ?
or maybe something else :?

ps tested on a system with 3 identical nvidia 9800 gt, for 2 different capture cards: lds falcon lp, datapath vision rgb