What's happening is your gearing is too short. The car is hitting the rev limiter, which cuts the gas for a second, allowing the speed to fall. Once you're below redline, the gas comes back on, the car accelerates again, and *wham* into the rev limiter again.
If it has the racing transmission, widen your gear ratios out until the maximum sustained speed, wide open, on the test track is just over the redline but not enough to hit the limiter. This is the horsepower-limited maximum speed of the car.
If the car is topping out but not reaching redline, that's drag-limited maximum speed. Shorten your gear ratios toward the "sport" end until you are just above redline, like above.
My turbo first-gen Miata went from a top speed of 132 with the stock 5-speed to a top speed of 184 just by installing for the 6-speed racing trans, and playing with the "auto adjust" gear ratios.
