This is one of the risks of taking a prep test too close to the actual exam date. The dip might have been due to many reasons. Too many silly mistakes caused by stress, mood, fatigue or quite simply even just a bad day where you went left more often than right.
So don't worry. As of now, we're already at the point of no return as far as rescheduling goes. So if you've been scoring 680ish consistently, then you'll be fine on test day.
Use the lower spike GMAT Prep to get some takeaways and formulate your strategy for Monday. Beyond that, it's best you forget about that particular mock test! (All this is just my opinion btw!)
This is something I'm telling myself these days (my exam is on Tuesday :p)..."GMAT questions have a certain elegance to them; try to enjoy it and just take each question one at a time!"
Hope you have a great exam...
Cheers!