This one has been stumping me for a while. Your input is appreciated!

Setup:99 RS, with HRC kit, stock fuel injectors + 440s in portfueler, 1:1 SFMU.

On mild acceleration, the car will lose power for half a second, then start accelerating again. It can do this several times in a row. Annoying.

A/F monitoring shows that ratio goes from 14.7 to 17 momentarily, when this happens. So ECU is cutting fuel, but why?

There is no backfiring. Fuel pressures are steady the entire time at 50 psi. TPS voltage is steady throughout. MAP voltage has no fluctuation, however when this occurs I've noted that the MAP voltage is generally decreasing as vacuum becomes more negative as I accelerate.

I thought that this may be due to a missed spark, leading to the front O2 sensor sensing an overly rich mixture, thus leading to fuel cut. But there is no sound of misfire prior to the cut, the A/F ratio appears stable prior to the cut, and I've removed all possible spark issue variables (new plugs gapped correctly, new plug wires, new coil).

I think the ECU is guessing to go into open loop/deceleration due to some kind of sensor inputs. But MAP, TPS, IAT, O2, CLT sensor readings looks fine, and car does not hesitate any other time so I use that to rule out Cam/Crank sensor issues. What is going on?