Only recently, I have been getting a tiny jump of lag every few frames. Attached is a screenshot of the alt+f3 graph. I have tried updating the current java version, turning all settings to minimum (which they are in the screenshot), giving it high priority with only essential background programs running, restarting Minecraft and restarting my computer.

The issue does not exist pre-1.9, performance is stable ~40fps in 1.8.
If I use fullscreen rather than just maximising the window, performance is also much more stable, albeit with much less frequent jumps.