LOST 10mph AND ABOUT 6 TENTHS

[Chuck Gallagher]

I agree, grounds are most likely weak.
Run a redundant ground wire from the side of the engine block back to the battery ground. Or, if your ignition box is inside, as most are, to the ground that is common with the ignition box and battery.
Use a good size wire and a star washer under the connector at the block.
Also, if you don't have a good size ground wire from the box to the battery negative post put one in.
Grounding to the chassis is always a crap shoot in a race car and the root cause for the majority of electrical problems.
Good Luck
Chuck Gallagher

I should have said this about engine block grounding. The most important task your battery does in your race car is provide a steady, high level of amps (current) to your ignition system. Racers in all forms of motor sports have been able to make more power over the decades by putting more fuel and oxygen inside the combustion chambers and completely igniting that mixture with higher power spark across the plug gap from the coil. The power (amps) of those sparks will never be greater than the resistance of the current path from the threads (ground side of the plugs) of the spark plug back to the negative post of the battery.
A weak engine to battery ground path does not always produce engine miss, usually just incomplete combustion resulting in less power.