Alright, here's an idea. Make an item-type variable array, where you store all your set item types (at map initialization, save each item-type to one index in the array). Then, you have your item spawning trigger, and there you do this:
Code:
create a itemTypevariable[(math-random number between 1 and (number of set items))] at itemSpawmPoint
Quicker than explaining what to do I've included a map which makes an item apper in a region every two seconds (just change it from 2 to 60)
This trigger should make any item, excluding the campain ones. To change the level of items created, change the 1 to 10 part of the line "Random integer between 1 and 10"
"Random of level -1" is any item, but it does include campain items. (level 0 items)