Whereas that most decidedly is a bit peculiar, it does not by itself explain the stutter. I mean, the statistics are fine as far as OBS is concerned. No serious framelag, no framedrop. You have a bit of an odd configuration though; 3500 Kbps max bitrate and a 4200 Kbit buffer. Why the custom buffer and why so large? And why that awfully high bitrate for the audio? 128 Kbps AAC is more than fine, most people can't even distinguish between 128 Kbps AAC and 320 Kbps.
And 18 FPS framerate? 18? 18 is a very non-standard framerate. 24 is a standard, 30 is a standard for streaming, 48 is another standard, 60 is the standard for 720p. 18 is... weird. Can I suggest reading my guide (signature), reading it step by step and just following it step by step?
As far as OBS not detecting 8 gB is concerned... not sure what to make of that one. May want to run DXDiag to confirm whether or not the 8 gB actually is registered to Windows in general. Start -> Run -> dxdiag