I realize this thread is a bit dusty but I wanted to add.
I have ran an FSX server for 3 years. I offer both Gamespy, and direct connection (to IP). I tried FSHost, but unless everyone uses it, they can not see each other. And the clincher, our operation utilizes the on board radio system, so it was useless to us.
After some investigation, I determined the cause of most of the problems with Gamespy. And it was only two very workable problems.
1) The server, must have the correct ports available.
This goes beyond the scope of FSX alone. One must also understand Gamespy and address its needs as well. In other words, a few more ports need to be adjusted outside of the FSX requirements.
2) The free flight lobby is too crowded and kicks people
Once you move the server to another lobby, kicks all but vanish. The system will return to your last lobby used at each visit. We use the Bush Flying Lobby for obvious reasons.
To date, the record runtime for the server without a restart is 61 days. I find it runs very nicely. We have events, such as Search and Rescue scenarios, contests, STOL Competitions, races, Poker Runs and other assorted events with little, if any, trouble. At times a lot of people are on and communicating. And I attribute this to port configuration, for both FSX itself, and Gamespy, as separate parts of the equation. And of course staying away from the Free Flight lobby.
Some do use Direct Connect, but it is a minority. Stop by, have a look on our forums. You might just re-light that multi player fire yet.
There are times that the system will report it has lost its connection to Gamespy, but I actually appreciate that. It kills the ability for the server machine to use the chat box, but has no affect on anyone else using it. They can still type to one another and use radios all day long. And, it removes the hassle of people sending friend requests and dropping the server into a "wait for a console input" mode which will hang it until it gets that imput. But I find using HippyHappy's request blocker does a good job of taking care of that slight annoyance as well. In our case, its an unattended console. The monitor gets turned on occasional for a restart, or a move to a requested airfield for an event. Thats about it outside of normal machine reboots. It is running on a P3, with a 128meg vid card. The graphics are set at min. and its staring at the ground. A speaker with a toggle switch allows monitoring radio traffic.
So, fly on a dedicated server with your friends. If it is configured correctly, it should work nicely for you.