Your car will get looser during a run. Tire Pressures come up which increases rear spring rate a tad plus your burning gas so loosing rear weight. A gallon of gas is about .5% of the car. If you can feel .25 of Cross, I'm sure you feel .5% of rear weight.
These cars run very loose to be fast. The winner is usually sideways off the corner for the second half off the race.
I think Freddy is on track, check your tire temps and wheel input. I know you run a lot more cross then I do so I would be looking into that tight/loose condition. Right as your on the gas to set the car off the corner the thing will snap loose.
IMO, a loose off race car is a more gradual feeling. But when your tight / loose, it just wants to snap around on you.