Mobs first light on fire then slowly swim up, magma cubes swim up a lot faster then the rest and don't light on fire. Item entities do not burn.
I am not sure how to reproduce it as I don't know what is causing it, I know it does the same for both 1.8.3 and 1.8.7 but I have not tested it in other versions
What's the setting of the world generator ? (Default, flat, custom ?)
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (
[minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]) here.