Most of the items that you recieve are totally random. Things like health packs and weapons are moved every time you start a new game. The only things that do occur at the same place are items that are given to you, items at the stores(though not all the time), and then items on ebon hawk and the beginning of peragus. Im not sure if this is correct, but i have been experimenting with the theory that you can find rarer items more frequently on harder difficulties, but i have not proven it so far.