Thanks for the vote of confidence, Chris.
The only thing I can think of is that you are falling victim to the computer's adaptive strategy. How you drive before doing a race run will determine how your computer responds. If you're cruising around driving conservatively to save on gas and then try to race, your computer will need to take some time to readapt for performance. If, however, you always drive somewhat aggressively, your computer will pretty much be race ready.
What you need to do is isolate a pattern. Is it sluggish under certain weather conditions? Have you not raced in a while? Did you use gasoline from a different source? Did you race recently with the nitrous on? Things like that. Maybe keep a notebook where you record the things you do and note when the car is running strong or running sluggishly. This might help figure out what's the cause.
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351W 89 Mustang GT Convertible