Why would oil sludge be in my radiator cap?

What is your question?

Car shuts off while driving after it has ran 3 or so miles but starts right back up and hear cracklings under the car when I get back home? 1998 Grand Prix gtp with the supercharged motor.

What makes this problem better or worse?

It’s just gotten progressively worse

How long has your 1998 Pontiac Grand Prix had this problem?

I think bout 6 months when I was given the car I checked the fluids n cleaned the cap when I checked the coolant