Lumpy engine when warm/hot

hi everyone, my first post, so dont jump on me if ive posted in the wrong place lol,

when first started my car runs ok, it doesnt miss a beat, until air injector pump turns off, then the engine becomes lumpy, popping sound from the exhaust, sound like the back box has no baffle,
when driving the car is fine at normal road speeds, just when the rev's drop below 2000rpm you notice its not running right, tick over is where i think it should be @ 950rpm, but its lumpy, ive changed the plugs, the leads, done a compression test, they are fine @ 210 - 220, a good dose of carb cleaner down the air inlet

any solutions please, its starting to do my head in, via pulling my hair out

the engine was rebuilt 3 years/18,000 a go, i have proof, fitted by main dealer

thanks tommy