@Francesco
You have a lambda based engine management, correct?
Did you followed the proper initial setup instructions?
https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/
If with correct a set up the issue continues, then ... attach a DMM .. to pin 2 to the ECU connector below the passengers seat and check if the voltage of the o2 sensor while your problem occurs and check the voltage, ... it should swing within 0.1 and 0.9 volts.
If not then check the duty cycle at the test port in the left engine compartment (also explained in the link above) ... I guess your ECU is driven mad cause of a non proper initial CO setup.