See e.g. this Wikipedia article on frame rate. Use 25 fps for regular sequence.
I counted at least 3 times where the picture flickered. How do I fix this?
It's hard to guess without knowing what's in those frames exactly. If everything is OK with all stills, there might be issues with your machine and disk setup in particular. Start a brand new project, import stills as a sequence, create 1080p25 sequence, drop your footage out of the numbered stills into the sequence and check if the issue persists. If yes, upload that project and stills somewhere so that we can check.
By the way, what is your spec: CPU, RAM, GPU, HDD setup? Where do you store your footages: on internal or external drive? Etc.