Problem Server Block Lag (Mindcrack v7) (Mac)

[Moosey2323]

I have a server that 3 of my friends play on. I run the server and use 1 gb of ram as we usually only have 2 people on at a time. However when we break blocks they always reappear and this is very annoying. Most of the commands (including the tick threading one) that I have found give my friends and I the following message when logging into the server.

Forge Mod Loader has found world ID mismatches
ID 343 is mismatched. World: (ModID: Minecraft, type uv, ordinal 2) Game (ModID: Railcraft, (message gets cut off)
ID 342 is mismatched. World: (ModID: Minecraft, type uv, ordinal 1) Game (ModID: Railcraft, (message gets cut off)
ID 328 is mismatched. World: (ModID: Minecraft, type uv, ordinal 0) Game (ModID: Railcraft, (message gets cut off)

I have run plenty of servers and played (on the same computer) including tekkit and vanilla and have never experienced this. I also have great internet speed. Here are my computer specs:

Mac OS X
Version 10.6.8
Processor: 2.53 GHz Intel Core 2 Duo
Memory: 4 GB 1067 MHz DDR3
Startup Disk: Macintosh HD

This is the command I have been using that works (lets us log in) but we have block lag.

#!/bin/bash
cd "$( dirname "$0" )"
java -Xms1G -Xmx1G -jar mindcrack.jar -o true