The four-times capped Welsh international midfielder revived Hammers' play-off hopes with an 89th-minute header, which left choked City heading home paying for their wasteful finishing and still embroiled deep in relegation trouble.
West Ham, who kicked off in ninth spot, made two changes from the side that had drawn against Sheffield United in the FA Cup last weekend, as Nigel Reo-Coker and Bobby Zamora came in for Luke Chadwick and the suspended Tomas Repka.
Following their 2-0 win over Burnley a fortnight ago, 19th-place City made just one change as Peter Thorne replaced Andy Campbell.
And the bustling striker soon forced Stephen Bywater into an early save before Junichi Inamoto's instinctive long-ranger forced the alert Hammers keeper - who was once on loan with the Welshmen - into another fine stop.
The Bluebirds' 4-1 win at Ninian Park back in November had equalled Pardew's biggest defeat as West Ham boss and certainly within the opening 45 minutes there was little to suggest that the Eastenders were going to exact their revenge as visiting keeper Neil Alexander watched a string of shots drift waywardly wide.
As the interval neared, Alan Lee - who opened the scoring in that Welsh walloping - almost repeated the feat when he ghosted into the box, before forcing Bywater to claw his angled ten-yarder into the side-netting.
Galloping away goalless at the break, City would have been far happier with their first-half endeavours and within moments of the restart Lee bulldozed through the heart of the Hammers' hesitant defence before Richard Langley's scorching 20-yarder was brilliantly palmed away by Bywater.
The bustling Lee was becoming an increasing threat and another barnstorming run saw him blaze over from 20-yards.
As the hour-mark came and went, Alexander still had not had a single shot to save and that was the catalyst for Luke Chadwick to replace the ineffective Zamora.
Midway through the half, unmarked 14-goal Marlon Harewood should have broken the deadlock when he rose to meet Hayden Mullins' searching centre but again his finish was way off target.
On 72 minutes, Cardiff should have taken the lead their performance deserved when Danny Gabbidon released ex-Hammer Jobi McAnuff whose cross into the six-yard box was scuffed by Lee, whereupon Langley's sliced six-yarder somehow ricocheted wide off Thorne's knee.
The unmarked Lee then drilled wide from 15 yards, and just as West Ham looked to have survived to share a draw, they somehow turned one point into three.
Fletcher sneaked in front of Gabbidon to meet Chris Powell's left-wing cross with a glancing 89th-minute header that crept inside the hitherto redundant Alexander's left hand post, to leave the battling Bluebirds literally on their knees.
External reports
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