I got this problem. I got a 17mbps connection, 1mbps up, well that's not the the problem

The problem is when i host it starts laggin immensley, mind you not all the time. Like i live in australia, and when i play with my friend who lives in the same city, there is no problem. When i play with my brother in the other room over the net, still no problem (+ my friend). Sometimes even on the net when playing with 1 person from the US it's ok. But when there are 2 people it ussually starts lagging. Like the ping SKY ROCKETS to like 50000 and over.
I have scanned my pc with everyhting, (adaware, win defender, norton) founnd some registry entries, and that's all. Didnt help.
Mind you when i just got spring, version before this one. I could host and download stuff, and watch online streaming tv on winamp.
Any help would be appreciated.
Regards,
Voz