1200 rpm is fairly fast so you might have the throttle blades open too far. Check your t-slot exposure like mentioned above. If your throttle blades are open too far then you should close them and either open the secondaries a small amount or drill holes in the throttle blades. You want the throttle blades set so only a small portion of the t-slot is showing at idle. Once those are set correctly then you can start figuring out if you are lean or rich at idle. Get the idle sorted out then figure out if you are lean or rich at cruise.
I agree... at that throttle position can you turn in
the mixture screws and kill the engine... you might be
open too far and are not in the idle circuit anymore and
make sure the pump starts to move as soon as you touch
the throttle.... and is it warmed up... cold it would
want to be fatter(think choke)
Light grey with race gas is probally a little richer than I would run it, try leaning the idle AFR down to see if that helps 
I've seen motors stumble both from being to lean and to rich 
