Low Fuel Light Goes On And Off ?why?

I Am Not A Mechanical Person , And I Just Bought A Used 2000 Gmc Sierra Extended Cab Sle * Very Nice Truck 49000 Miles Other Than The Low Fuel Light Keeps Going On And Off Even With A Full Tank * It Always On Til You Get Up Some Speed *the Mechanic At The Dealer Said Fuel Sensor? Help . Me Cuz It Seems To Do This When Their Is A Change In Electricity Used As Well? Am I Gonna Be Ok Or Do I Need To Sale * The Change To A Higher Speed Usually Goes Out * The Simple Task Of Turning Cd On Will Sometimes Make It Come On * Your Help Appreciated I Think It"s More Electrical ? Thanks Jd Of -jd Freight Inc

have it checked for fuel level sensor codes, sounds like a fuel level sensor problem, they will usually set a code when the contacts are defective.