Saints' top-scorer smashed in a 50th-minute penalty before teeing up centre back Jose Fonte to head in the second eight minutes from time.
The visitors, who slip to 15th, played almost all the second half with 10 men after captain Sam Collins was sent off for bringing down Lambert.
Lambert dusted himself off from Collins' tackle to convert his 19th goal of the season.
David Connolly was denied by the impressive Scott Flinders before centre back Fonte climbed to meet Lambert's floated free kick.
Source: PA
Source: PA