Sounds to me, too, like this is about the DAW (Reaper) and not BIAB.

I have a similar configuration.
Occasionally, I will get a "stutter" in the playback in my DAW (Studio One Professional)...

It is almost always because of the Windows Update routines (svchost.exe - a couple of them usually) running - downloading stuff and hogging memory. Once they finish - and go away - so does any playback problem.