I have a 2012 IS250 with 30,000 miles on it. I have been experiencing this intermittent issue since Oct. 2015. The problem begins about the same spot in the gas level which is about 3/4 of a tank. When I start the car it idles very rough like it is wanting to die. When I reverse out of my driveway it barely moves, and then I put the car in drive there is no power to accelerate. I have no power and will barely move until I reach 45 mph. This happens at all stop and go situations. I cannot pull out into traffic unless there is no cars coming otherwise I will get hit. The delay lasts about 5 seconds just sitting in traffic waiting for the car to move forward. I can put the pedal on the floor and still just sit there until I take off like I hit a boost button. I have taken the car to Lexus on multiple occasions to fix this issue. The first recommendation was to clean the fuel system, and that didn't work. The last few times they have kept it for a few days and they couldn't get the car to reproduce the problem. I asked them to check the EGR Valve and that wasn't the problem. They advised me to keep a log of this issue so we could open a case with Lexus. Each time this circumstance occurs it lasts for about 45-90 miles. I have had no check engine lights come on until the other day, which was a bad O2 sensor. They kept my car and drove it a few days and the car finally had the acceleration problem occur. They called me today and suggested that I pay them to remove my gas tank from the car and clean it and claim "bad gas" is my problem. Clearly it isn't plus they want $1400, nope! I have sugguested to check the throttle body and some other things and they do nothing. Frusterated. I told them to find a better solution and get back to me on that. Still waiting for a call.

 

Sort of sounds a little like carbon buildup. Have you tried resetting the ECU? Maybe the car has learned sluggish driving habits or something.

 

My car is still at the dealership. I will suggest that to them, you would think they would have tried that. I have no clue on why they cannot troubleshoot this better.