If you don't make 5 reps for 3 sets then you just repeat the weight the next time you are scheduled to do the exercise. If this happens 3 times in a row ,i.e. at the same weight, then it's "a true stall" and you reset.
So you do 83, and if you don't get 3x5 you try it again next time. And if you still don't get all 15 reps, you try it again. Only then would you say you stalled.
(Personally, i tend to give the pres specifically a few more attempts, but that's just me.)
(Did you ... read the book? search the site?
"what is a stall" is addressed a good number of times. Though the SS book is a little vague on the specific protocol...)
edit: also, make sure you're incrementing your DL by 10lbs to get it out ahead of your squat. What specific program are you following ?