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Why the WiFi adapter continuously changes its speed is something I can not answer, but the "TCP previous segment not captured" and "TCP dup ACK" messages can be explained.
In your screenshot (please supply a network capture file next time, it makes analyzing so much easier), frame 36036 mentions that a segment of the TCP stream is missing. This is probably caused by the switch to a lower bandwidth in the middle of sending a packet. All the following data packets are triggering a duplicate ACK as that is the way TCP tells the other side there is still some data missing.
Then in frame 36080 the segment that was lost is being retransmitted which stops the duplicate ack's as there are no more gaps in the data.
Nothing wrong in the pcap file, except of course that packets are being dropped due to the switching in WiFi speeds.
To be able to analyze the reason of the WiFi speed changes, you might want to capure in monitor mode on another system to see the WiFi management frames. This will tell you signal strengths, amount of retransmissions at the WiFi layer, etc.
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