My engine runs like a sinusoid

Hello,
I probably have a similar problem as explained in the post "Engine go up and down in rpm "somtimes" .
I've a chipped V8 with 3.6 PT engine and 5-spees manual gearbox (250.000 Km). While I connect the car to VAG-COM I find no faults. The "CAT" lamp goes off after I start the engine.
If I push the Throtthe pedal at about 1/2, for example in third gear, the engine hasn't a constant acceleration but accelerates more an less like a sinusoid. This problem isn't very evident and the car works almost fine. I reached 265 Km/h on the tacho but the car was able to accelerate more.
What do you sugest to check?
Idle control valve? (When I start the engine the rpm oscillates for 2-3 second before idle stabilization)
Lamda sond?
Mass air flow meter?
Fuel injectors?
Fuel Pump?
Fuel pressure regulator?
Anithing else?
Thanks in advance for your advice.
Nicola
Italy.