CPU System agent core (TC0E/TC0F) running very high
I would do a SMC reset (google on apple)
In addition to iStat, i have Macs Fan Control.
just use iStat to DISPLAY fan speed and CPU gfx core temp
Use Macs Fan Control, MFC, to control fan speed.  By default, my mbp's fan will not come on, and often doesn't even when way beyond 45C!
in MFC drop down menu... Right Side - Sensor-based... Change control...


Sensor-based value: GPU PECI
Temp that fan speed will start to increase from: 30C - 80C
OK to confirm change

I believe MFC is a linear fan curve, as temp increases, fan speed will also go up.  I prefer it this way.  Lost my last mbp to lousy fan control, not happening this time.
my MFC just shows fan icon in menu bar, but no fan speed or temp info.
Remember in MFC's preference... set it to autostart


Im out and about quite a bit with my mbp.  I use a hard shell case from iPearl (search on Amazon... our model is A1502).  Its got little legs I can prop up the rear of my mbp, to keep it cooler.  Fits well, no issues here, I am typing on it right now.

mbp 13" retina 12,1 blah blah,   ~1900 rpm, 35C

As far as CPU System agent core on my mbp, iStat will report it to be 0 at times, then it'll resume back to normal in a sec or two.  Let me know if u figure out that one.  thx.