I have a 99 camaro lq9 block, cam, 241 heads. Runs fine will let you rev and idles fines but once at operating temp idle gets choppy (it lobes) and cannot rev it acts as if it's loading up or leaning out but still idles. Will do this at idle or going down the road it acts like you turn on a switch and The loading up begins. Have good fuel pressure and brand new walbrough 255 fuel pump put in after this started and the problem continued. any help appreciated.

 

Could be a couple of things like 02 sensor problem or maf. When car is cold it is in open loop(02 sensors aren't working) then as it warms up (on mine 150 degrees) car goes into closed loop and starts using the 02 sensors. If you have a way to watch 02 sensors you can see if they are working correctly.