It could be a number of things I suppose, but I'd say first you should check your pulleys and second, read up on the idle control valve.
Usually when I hear of an idle being off, I'll think idle control valve. Ours seem to get gunked up a lot and that prevents it from controlling the idle as it should.
You should idle around 700.
When you turn the steering wheel, it puts more draw on the engine, especially if you're turning while not moving.
You may have a leak in the lower intake (most common leak) too, but it could be elsewhere.
Anyway, read up on icv, and if it sounds like that might be it, you should probably clean everything in your path, including TB, disa, and maf.
How many miles on your car and what work have you done to it already?