The game will crash a little after the world opens, this is a world we have been playing for a while and all of a sudden this started happening, no changes were made between the pack working and it not working anymore.

client crash log: http://pastebin.com/CUX7s8Z6
server crash log: http://pastebin.com/uEdYL4aM

I have a similar issue, same crash logs, although I'm making a modpack, in mine the players get disconnected from the server and it hangs for quite a while, I attempted to force a stack trace with jstack, but it returned saying the process id didn't exist, checked screen, no running processes, so I checked the crash log folder and sure enough there it was...

Although I expect this sort of thing when cramming a ton of mods together...

edit --

forge-1.7.10-10.13.4.1558
NotEnoughItems-1.7.10-1.0.5.118-universal.jar
CodeChickenLib-1.7.10-1.1.3.138-universal.jar

Modlist... (mod folder dump, first couple are directories)
http://pastebin.com/AzP0gFGf

There are a few unshown ones in the 1.7.10 directory, but they're almost certainly unrelated...

I should note this modpack is presently unpublished, and as such I haven't gotten permissions for the mods yet, that'll happen once I finalize what mods will be in it in the first place...

Assuming this is due to using the latest instead of the stable version I'm going to try going back to stable (and downgrading Waila to whatever version works with stable)

Update --
Can't downgrade waila, as (I think it's Thaumcraft, but not sure) something requires the latest if it's installed at all... Which means I can't downgrade NEI to downgrade CodeChickenLib >.>

Another update --

It seems to be related to NEI since it doesn't occur when it isn't in the mods folder. But the stack trace on a crash during the lockup occurs in CodeChickenLib code. So not sure what's going on here... It's tough to debug because it only seems to occur when loading (unloading) new chunks, and then intermittently, so I think it'll be uncommon enough in normal play that I'm just going to use these versions, and remove NEI from the server-side. On testing the client still gets disconnected but the server is unaffected and the client can reconnect immediately.

Another possibility is that these are two different issues that just happen to coincide with my modlist...
The server crash and the (seemingly) infinite loop somewhere (causing the disconnects)