Before you start, make sure the other player has the same ROM as you. Most people will keep the ROM in its zip file, while others will unzip it and have the SMC file. I believe the reason why the game freezes is when 2 guys try to load a ROM at the same time. I think the person hosting should load the ROM and make sure you let the other guy know that you are loading the ROM so this doesn't happen.
If they don't have the same version of the ROM you have, you can always send it to them though AIM.