Welcome aboard!

LB7 or LLY (8th digit of VIN, 1 or 2, respectively)? How much surging? RPM's? What's the cadence? Can you see it on the tach, or just feel/hear it? Is the SES lamp on, and if so, have you pulled the code(s)? Tailpipe smoke after long idle? Have you checked for air intrusion (leak in new fuel lines/connectors/filter)? Which fuel filter is installed, steel or plastic? If plastic, they can leak w/o any help (toss it and use OEM or quality steel).

Rusted/rotted fuel lines can certainly cause contamination. Once any part of the system is compromised, the entire system is suspect. You need to get a scanner on it to see what, exactly, is fluctuating and causing the surge (check command/actual pressures), and check for any DTC's active or in history. Any significant pressure fluctuations or inability to reach/hold a pressure will set a code, and likely the SES lamp if it repeats.