engine cutting back
 

hi,

I have a 1997 Z71 with a 5.7 350 ,i just put a new engine in and engine is running fine ,except for one problem.
When i'm going down the highway at 45 to 50 mph and accelerate the engine starts cutting back,it only last for about 4-5 seconds then smooths out,and it it only does it in overdrive. I took it to the guy who rebuilt my trans and he said the trans was fine. He hooked it to his computer and it threw the code "fuel mixture too rich in bank #2".He also did a stall test on it. Any help would be greatly appreciated.