Hey,
When it wasn't asking for the serial key, this is because there is a hidden folder in the C drive called ProgramData. Inside it, there is a Test Drive Unlimited folder, with a notepad file with the game serial key you inserted before. It stays there until you delete it. When you install the megapack, this creates another notepad file with the megapack serial key, so no matter how many times you re-install TDU and delete the remaining files, it will think you have the megapack installed because of the serial key in ProgramData. So if it wasn't a legit serial key from a megapack you bought, gamespy wouldn't let you online with it, so like I said, no matter how many times you re-install, gamespy will still think you have the megapack installed by the serial key in ProgramData for the megapack.
If it was doing it before you installed the megapack, that's down to your computer, your internet connection or the gamespy servers.
And yes, the reason your save game will not work is because it has megapack data in it. Megapack data - Megapack = Random useless data that won't work.
Hope that helps, and welcome to TDU:C! ;)