Check the pipe just before the throttle body for splits, especially in the grooves, they get dry and split. This will cause exactly the symptoms you are describing.
I own a 1998 156 2.5 v6, and I have the same idle problem. I don't think it is the split on the corrugated rubber pipe. If there is such leak, then the idle has to be higher that usual..because there is more air getting into the mixture...
I have cleaned my throttle body several times. I have had my MAF and lambda sensors replaced with genuine BOSCH parts...I've had my spark plugs replaced with NGK ones. I fitted new air filter. Oil and filter change done... But still have this erratic idle problem. I am sick of it. Sometimes the engine stalls in a parking lot..sometimes at a speed bump.. It is becoming really dangerous.
I suspect the culpit is the small stepper motor on the side of the throttle body.
Does anybody know if I could get only a new stepper motor without buying whole throttle body..?
All the best.
1998 Alfa Romeo 2.5 V6
Metallic Silver Grey + Black Momo Leather, 17” GTA wheels, 215/45/17 Kumho Ecsta KU31, Full Zender Body Kit, Brembo Disks, Ate Brake Pads, Bilstein B4s, Vogtland -40mm, Supersprint Muffler