Art61

I have a 1995 Chevy truck new engine crate when I start it when it's cold it chokes and gags no check engine lights engine coolant temp sensor new when I plug a tech 2 scanner into it The tachometer in the truck show is about 900 RPM the desired idle on the scanner is about 900 RPM the actual idle on the scanner is a Erratic going from 2600 to 1700 down to 500 truck is choking itself unless you push throttle it will just sit there and gag for about a minute and then clear up any ideas why the computer would think the engine is doing 2600 RPM when it's not also once warmed up it still doesn't run good stalls sometimes been to two GM shops can't figure it out I'm at my wits end

thanks art