I've replaced mine once, but it wasn't necessary (found out it was the EGR regulator causing problems instead). What a lot of people don't realize is that the EVP (EGR valve position) sensor that bolts onto the back of the valve has an O-ring to seal in the vacuum. When the dude swapped valves, I bet a quadrillion bucks he didn't replace the O-ring OR the sensor, and just bolted on an entire 'assembly' without checking it first. The EVP itself could also leak. You could buy each item (valve and sensor) then experiment at home to determine what's going on. There's always the real good possibility that yes the EGR valve IS bad.
This guy bought USED EGR VALVES?!?!? WTF?!?!?!
Keep your eyes open for other strange things on this car, dude.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch