OriginalContentId holds the linkage between item's lifecycle versions (Live, Temp objects linked to Master one).
If OriginalContentId is Guid.Empty, that means that the item is in Master status.
If OriginalContentId has a non empty Guid value, this means that there is another item (Master), which Id is equal to the OriginalContentId value.
You can add filtration to your randomizer by Status.
Let me know if this helps you resolve the problem.
Although all the products are published and live, they will still have a master version of the item, which means you will have items with OriginalContentId == Guid.Empty as many as you have actual items (live + draft).
Make sure that you check the for ContentLyfecycle.Live and for Visible == true in the query. Following is an example:
OriginalContentId is actually the Id of the master item. It is the connection of live and temp items with the master item. So, live and temp items have OriginalcontentId == Id of the master item.
Master item, on the other hand, has an OriginalContentId == Guid.Empty, i.e. them master item is the actual original content.
Moreover, the Id of the live item does not change if it goes through another publish cycle.
About your case, I suggest you to get the live items and store their Id instead of the OriginalContentId. Later you can retrieve these items by that Id.
If this does not help, please send your code so we can inspect it in details and figure out the best approach of the problem.