Every so often the back of our gets into bouncing or hoping and does this down the straights.
Car is a 4bar Zlink.
Could it be the rear shocks heating up?
Could it be the driver getting on the gas earlier and then it climbs on the bars faster and runs out of travel?
When it doesn't bounce the car is a front runner.
It could be the axle tube hitting the underslung bar, or the shocks are bottomed out. I would make sure that your ride heights are correct. I know a few guys that had their ride heights too high, and it would wheel hop down the straights due to the axle tube hitting the underslung bars.
Drop your LR upper link a hole at a time until it lifts smoothly.The car is lifting too hard (or quick) and you are either on the frame or the end of the shock travel so your tire ends up being your suspension.(The old slide hammer action).Everything needs to work smoothly to be fast.
you've probably got a rabbit on board somewhere.