Ok, here's what's happening:
I made my own personal modpack with a collection of mods. I start up the game, and everything is fine, the menu works properly, and the world generates correctly.
When I start playing, however, things go wrong.
A short time after the world generates and I start playing (around 10 seconds or so), the game lags/freezes. I don't know what to call it.
What happens is I can still walk and run and jump around the world and break blocks, but I can't interact with the world.
List of what happens:
The chunks don't load.
I can't hurt mobs.
I don't take fall damage.
I can't interact with blocks (chests, furnaces, etc.)
I tried to quit the world, but the game keeps on getting stuck at "shutting down internal server", so I have to force quit Minecraft/Java.
The first time I encountered this, I removed a mod and the modpack worked perfectly. I never added that mod ever again. Then, I added some new and different mods, and the problem came back, so I removed those mods. But the problem still persists to this day.
Any help? This problem leaves no crash report.