Some questions about an 04
 

Just picked up my new to me 04 3500 quad cab long box. Trying to learn as much as I can as this is so much different than my 97 12 valve.
First off, it came out of Texas and was used as a hauler. Has banks power badges on the front quarters, so I'm trying to figure out what all mods it has. The obvious is the banks exhaust. Has a new holset turbo and the head has obviously been off. It does have only one issue I'm trying to resolve. It developed a bit of hesitation after driving it for about 30 minutes. It puffs a little black smoke when it hiccups, and it only does it when it's in between 800 to 1400 RPM. Stiil pulls strong and runs down the hiway great, just the off idle problem. Scanned it today and the voltage to the injectors was steady, the fuel pressure was fine, showed no codes.
Wondering if it's been tuned and that is causing the problem. How can I tell? What do you guys think?