MovieStim seeks are always slow and imprecise. See here: MovieStim (python) seek behavior: ~5 frames to complete
MovieStim3 won’t solve this issue either, unfortunately. It’s just unreliable about seeking in general, and I never found a solution I was happy with.
I did come up with a solution that is at least somewhat reliable, which you can read about here and in the repository that this post links to: Using seek with MovieStim - #2 by jonathan.kominsky
The solution I came up with introduces about 100ms of delay into the process, but it’s a reliable 100ms and it can tell you exactly when the frame you want is actually presented. However it is not trivial to implement and involves stupid tricks like playing the movie behind an occluding image. If you didn’t have audio I would just recommend not using movies at all, but since it sounds like you are actually dealing with similar stimuli to what I was working with, this might be your best bet.