Optiplex 320 + Radeon 5450 Graphics + Windows 10 == video lockup!

This one is pretty odd.  Wondered if anyone has any clues as to whether it can be solved:

We have an Optiplex 320 computer that we use for streaming video on a HDTV.  The system was given to our son, who was paid staff for the 2008 Obama campaign, when the campaign disbanded.  So this is one time "Thanks Obama!" has some actual applicability.

Anyway, it can handle HD video streaming -- just. I added an ATI Radeon 5450 graphics card because the built-in video couldn't stream decently.

I did the Windows 10 upgrade when it came up in Windows Update, and in general it works fine, with one rather vexing problem: When I stream video from Netflix, either in the native Windows 10 Netflix App, or in the Edge Browser, it locks up the computer after playing for a few seconds.

Has anyone else run into an issue like this? Is anyone else still using a 320?  The ATI SB600 motherboard chipset was definitely an evolutionary dead end -- I could never get Ubuntu Linux to install, either.

I wouldn't even bother but it was a free computer, and it still does the job.  It seems to run much better under Windows 10 than it did Windows 7; the work Microsoft did to get Windows working on lower-spec machines has really paid off.

I notice that Dell has pretty much disavowed this model entirely at this point.  Maybe 4 OS versions since the one that came on the machine is too many.