Tuner is AverMedia AverTVHD Duet. Antivirus is turned off. Windows is up to date (windows 10). Uninstalled and reinstalled JRiver multiple times. The video does not stutter in any way using the AverMedia-supplied software. Uninstalled and reinstalled AverMedia tuner drivers multiple times, no change. This is a dedicated Windows 10 media system and has no other unnecessary software installed.
Given that (1) the Avermedia driver and media software work fine, and (2) JRiver is using the same video card as AverMedia but JRiver video stutters and AverMedia doesn't, that points (I believe) to this being a JRiver problem.
It simply isn't worth investigating any further, unless JRiver support has some magic bullets (monitoring or instrumentation that points to this NOT being a JRiver problem).