Also be sure the car is in Drive and NOT OD. If it is shifting into OD at 99 mph, you will not go faster than this in the 1/8th mile.
Also if you are running a 93 chip, it will cut timing back in Drive, which will hurt MPH.
Any Knock? 80 mph is a little slow for an 8.40 so the low mph is showing up after the 2/3 shift.