Conditions can change over the day, including household traffic, wireless use and the path to a source; timing alone does not identify which one changed.
Why this happens
An evening-only pattern is useful evidence, but it is not a diagnosis by itself. More people may be using the household connection, nearby wireless networks may be busier, or the route and source may be under different load. A morning speed test cannot describe all of those evening conditions. The important comparison is what happens on the actual viewing device with the same source when the symptom is present, rather than a collection of unrelated tests taken at convenient times.
Choose two short observation windows and keep the source, player and device the same. Note household uploads, calls or downloads during each session. If possible, compare one supported wired route during the difficult period so the local radio link can be assessed separately. Google’s network-performance guidance distinguishes internet speed, device capability and local Wi-Fi conditions, which is a useful way to organize the result. Avoid promising that one router change will solve a recurring pattern before the evidence shows which part of the connection is involved.
A useful way to work through it
Compare the same source on the actual viewing device at the two relevant times.
Note major household network activity during each observation.
Test a supported wired route during the problem period if practical.