The most obvious diminishing returns addon is called DiminishingReturns (v1.0 beta 12 is what I'm using and it works fine)... it's possible that your main issue is the mess that is the combat log; get combatlogfixer as well (or be diligent in running the script manually).
BTW, why 18sec? Retail had between 15s and 20s (15s timer + 5s check intervals) and every single private server I've been on has non-blizzlike pure 15sec DRs as far as I am aware, even AT/Blackrock worked in that unblizzlike fashion last time I looked.