I don’t like using programs creating artificial stress situations since often you see symptoms which are not the one you want to inspect and you start hunting phantom problems.
Nevertheless I tried your test setup WITHOUT Deluged installed, running a a Full HD mkv with DTS-HD MA (passthrough) and the example parms for stress like you did.
It runs now since 38 mins without any problem. I see in “top” 14 stress processes running and taking an overall CPU utilization of constant 100% (60%user, 40% sys), around 700 MB used memory, 200 MB free.
With the actual stretch development repository you get a kernel which allows you to run iotop. While my test running, I see nearly constant throughput about 1.5 MB/s from the Filecache/HDD.
This is just to compare your test scenario and circumstances with mine in case you want to repeat it.
So, the good question is: Was this Deluged stuff running while your test? And if yes, what happens if you deactivate it completely or even deinstall it and repeat the test?
You should see nearly the same CPU utilization, same memory demand and behavior like here.
Btw. the video playback and stress processes were still running without any issue while writing this last line …