"One- way" lag - Extreme latency from server back to client

[efstajas]

I just set up a local server on my pc to play with some friends and port- forwarded the ports in my router. Everything worked great for some days, but then suddenly we got extreme "one- way" lag. What I want to say with this is that when someone types, for example, I receive the message in the console directly, but it takes several minutes (!) for the message to appear back in the client that sent it. I don't have any lag playing on the same machine that the server is hosted on

My PC is fairly powerful, and I even tried to allocate 4 gigs of RAM to the server, which didn't make any difference at all. My friends' internet connection isn't extremely powerful, but it's not bad either. My own internet connection is quite top of the notch since I live in the city center.

Are there any mods known for causing lag? We mostly use Redpower - we had our first quarry sorting and refining system powered by Buildcraft, but we noticed these pipes created massive fps drops, so we switched to redpower. We have a central distribution pipe which has several routing options based on color and during intense times there can be several stacks of items in one pipe at a time, although reducing the sorting system's clock speed didn't make any difference either.

We also use Mystcraft and have a nearly stable world with dense ores that has a quarry in it.

Another thing that I could imagine to cause some lag is a pretty decent animal farm which is being fully automated with mine factory reloaded blocks, and collects quite a bit of sewage through buildcraft pipes.

Now before I start to dismantle our base to find out the cause of the lag, are there any known problems that might cause the lag? Should I disable anything in a preference file to boost the performance?

What I tried already:

• Disabling IC2 power network by disabling the main transformator with redstone - No effect
  
• Disabling the redpower tube distribution and sorting network by disabling the redstone pulse of the system - No effect in latency, slight fps boost, many overflowed chests
  
• Breaking all dimension anchors (we only had 2 of those anyway) - no effect
  
• Deleting the lagging players' player- file - no effect
  

Is there anything more I could do?

Thanks!

[The White Ninja]

I am having a very similar issue on my very similar server. I have 12gb allocated to my tekkit server, and I have no trouble when I play on the same machine as the server (could this be a cause? should I try to use my laptop to play when others are on also?). There are only about 3-6 of us on max at any given time and if myself and another are on alone there is no problem, it is only when a third joins that there begin to be issues (not for my end but for theirs, the 'one sided' lag situation).

I have not tried disabling any mods yet, I am running the standard Tekkit Lite lineup with just a server side Optifine 1.4.7 manually added.

Anyway I suppose I am just adding a 'Bump' here so someone can help efstajas with his issue, because any response will likely be helpful to me as well.