Large amount of [TCP ACK Unseen segment]

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I'm having problems with streaming video on my home network for quite some time now. Often, the streaming video will suddenly and randomly freeze, so we need to restart the app/stream. This happens on various devices and streaming providers.

I tried various solutions in router settings etc., but nothing seems to have helped. Therefore, I decided to take a look with Wireshark. Now, what surprised me was that there's a very large amount of ACK Unseen messages. A subset of my capture can be seen on cloudshark. This is a capture of a stream from my girl watching Netflix on her iPad, after she just had to restart the thing because the stream had frozen. The ACK unseen messages keep going on, it is just a limited subset of the capture.

The router does not seem to be under heavy load, as the CPU rarely gets above a few %. Also, there are no strange messages in the router's logs.

Now, is this normal? Is it due to the router-Wireshark connection not being able to get all the messages in a stable manner, or does it indicate a problem with the streaming media indeed? Could it be that the firewall on the router is blocking too much packets (due to incorrect configuration maybe)?

Any ideas? What could be the problem, and even better, a solution?