Use something like a slightly different background color to signify which are random so users know that they should expect those to change with each page refresh.
If a rant that would naturally fall on the page is randomly chosen, just choose another at random to prevent duplicates.
This also opens the door for some more gamification items to influence your rant's probability of being chosen for these slots.
Don't call them random slots. That sounds bad and we'd have to explain it. Instead, just say it's part of the gamification - the chance to get your rant seen.
Have the order as expected, but do something like
1, 2, 3, 4, random, 5, 6, 7, 8, 9, random, etc...
Use something like a slightly different background color to signify which are random so users know that they should expect those to change with each page refresh.
If a rant that would naturally fall on the page is randomly chosen, just choose another at random to prevent duplicates.
This also opens the door for some more gamification items to influence your rant's probability of being chosen for these slots.