code that comes and goes

[bird of fire]

about a month ago I go this code P0153 Heated Oxygen Sensor (HO2S) slow response Bank 2 Sensor 1. I put in a new 02 sensor and that code came up again (p0153).I cleared it and got this code this P0174 Fuel trim system lean Bank 2. When I got the lean code you can smeal the unburnt fuel. I put in a mid grade fuel instead of regualr and the code went away for 2weeks. Then I got this code again P0153 Heated Oxygen Sensor (HO2S) slow response Bank 2 Sensor 1. When I started the car now it's idle is a little higher then normal by 100 to 150 rpms.When the car heats up the idle is normal. This all happens after i got rid of the egr vavle I put the block off plate and did the simulator too. Some one please help!!!