Random lag. Modpack issue?

[Zet0r]

I have an annoying issue with seemingly all modpacks I make. I host a private server for me and my friends, using a special hidden modpack that I've made. But here's the problem:

While I play perfectly fine, my friends are all experiencing lag (the internet kind, not FPS drop) of about 20 seconds delay, or at least 20 seconds until their chat message pops up for them. Although it appears instantly for me and the server GUI. So basically they type something in, I recieve it instantly, then they first recieve it 20 seconds after.

But it's not my internet, because another Modpack we've tried (The Lapito's Galacticraft Modpack & Server) seems to have no lag at all! Only like half a second in the very beginning they log in!

We've tried many things, even removing all mods except Galacticraft (which works fine on the other modpack), but it still lags. We've tried resetting everything and re-modded the server jar file. Could I have done something wrong in the way of making a modpack? Or something about the server?

Details are as follows:

Modpack Setup:

bin -> modpack.jar (minecraft forge zip renamed, yes it is a jar file)

mods -> All the installed mods (See below)

coremods -> Galacticraft, NEI and CodeChickenCore

config is not there, because nothing has been modified as of yet. We have tried a copy os the servers config folder as well as the generated client's config folder.

Mods Used:

All mods in the Universal Electricity Pack (Just like Voltz, without Blastcraft and Railcraft)

Smart Movement

ExtraBiomesXL

Mo' Creatures and the required side-mods

PowerCraft (Makes some error in the log, we have tried without it but it doesn't help)

AtmosMobs

Thaumcraft

Necessities (Server mod only, /home, /tpa ect.)

RedPower

The RC Mod (We have not been using it lately, so it has been tested without)

Balkons Weapon (Same as RC Mod)

In the later version we have removed all mods that are not part of Universal Electricity. We've also removed ICBM and Fluid Mechanics since they seemed to crash the server. In the latest we only have Galacticraft and the NEI package.

All mods are of the newest Green Build (Recommended) version, all fitting 1.5.1

Server is hosted through Hamachi, but that can't be the issue since the LGM-pack works fine.

Server has been getting a RAM range of 5096 to 4096 MB through a .bat file.

The server is the vanilla minecraft_server.jar modified with the latest Recommended Build of Minecraft Forge and when using Smart Movement it has Player API as well (Installed AFTER Forge).

Minecraft Forge Version as of now:

minecraftforge-universal-1.5.1-7.7.1.611

What could possibly be wrong? :/

[Zet0r]

It seems as if no one know of any answer. If someone who is familiar with making mod packs could tell me if it is the modpack at all or just random internet problems every time we use those servers? Or is there an issue with the pack? I'm going to try experimenting with it a bit more and see if it resovles itself. I just can't see what's wrong, it's basically a mimic of the Voltz pack setup only with other mods :/

[LazDude2012]

Hamachi's likely the issue; port forward it and host it normally.

[Zet0r]

Hamachi's likely the issue; port forward it and host it normally.

I tested it for a bit and it seems that there are no issues with Hamachi. All other modpacks that I haven't made that we've tried do not lag. Hamachi reports a 40-60 ms ping if I ping my friend. I tried comparing some configs but still nothing mismatch :/

Also he appears to be kicked out with End of Stream. The server log says it's an overflow error. But he can play Single normally without any problems.