Fuel Injection problems of the 2006 Toyota Scion Xb

Sometimes when I need quick response power from the engine it doesn't not have power to accelerate. I have to step on the accelerator a few less than full and the vehicle run like a turtle and after 4000 rpm the power start to come. This is a problem at intersections, traffic lights when I really are a low speed car at traffic when start to run. Also the automatic transmission change up to overdrive at 35 mph too quick getting worst the acceleration when you are normal driving. Is not "check engine" indication but one automotive mechanic after he drive says than the engine have a bog problem too lean fuel mixture. When he turn off the a/c the problem is less he says is because the idle control valve don't by pass air at throttle and it's less air getting at the engine without tps registration.