||CAS all in good working order. I had a similar dip in throttle response that I traced back to a bad timing map in an after market EPROM.
The first few rows in the map were 5' timing, and you can guess what happens when you come off 15' ON Idle and immediate fall into the fives. Once I passed 1200 RPM, everything was OK. Also the same issue with a cold start because the ECU is in closed loop, effectively enhancing the problem. When warm, it was better, but still present.