2006 bmw x5, gas meter needle goes from empty to full nonstop back and forth

Hello, an assumption that the pump itself is the culprit causing the problem is merely a guess; the fuel sending unit may or may not be part of what you replaced. Further, wiring concerns can cause what you are experiencing -- if there is a shorted wire or a wire with bare insulation grounding sometimes it can cause fluctuation of the gauge. Hands on testing with proper test equipment to monitor component operation and see what happens when you manipulate the wiring harness is needed. A voltmeter / ohmmeter will be helpful in circuit testing wires. www.repairpal.com/repair-shops