Have wireshark save the capture-stream at a certain amount of frames, then start new capture

"You can specify to use Multiple files during your capture options. From within those options you can define how many capture files you want and how large each file is. So you could say specify to have 20 files 20MB each. And when you're doing the capturing, Wireshark will do a round-robin of those 20 files and every time it hits 20MB, it'll go on to the next file.

Once you stop the capture you can take contiguous files and merge them chronologically too if you wish.

Depending on how much data you're doing, you might want to increase the buffer size a little too, to make sure you're not dropping any packets while capturing."