Jerking/shaking under 4krpms?

[Hexxy]

(sorry mods if I put this in the wrong part of the site, please let me know if do)
I am a young car enthusiast (15 and a half) and I just got a 91 S13 with a sr20det (350+hp). I was driving it for the first time yesterday in first gear around my neighborhood, when I let off the gas anywhere under 4000RPM's the car jerks around, the guy I bought it off of says it has new hardened motor mounts so its probably not that, he said it probably needs a tune though as he only flashed one he found on the internet. Also when its jerking like this the air/fuel ratio is around 11.5/12 and usually its around 14.5/15 (idk if this matters.) Please help, i'm worried something is seriously wrong.

Also it has a stage 5 clutch so maybe I just need to get used to driving it? I'm not sure.

[SupaDoopa]

Maybe you should tell us exactly what the car has for modifications, any weird smells/noises, if you can actually drive stick, pictures of this thing.

[deolio]

you drove around and just left it in first gear the whole time? lol did you put the clutch in when you let off throttle or leave it engaged?