bucking occurs after every gas fill up and then occassionally on and off after that on 2001 Nissan Sentra

Weekly the car bucks and always after fill up

2 answers
I wonder if the canister purge valve is sticking open and when you refuel the intake manifold is filling with fuel vapors. I just looked there are two service bulletins from Nissan about the evaporative system having problems.
I dont think the bucking is associated with the gas fill ups. Sounds like you have a secondary ignition problem that needs to be fixed. (cap,rotor,wires, fouled sparkplugs, etc.)