[fast924S]

HAte to say it but it sounds to me like a head gasket. The idle is stumbling cuz the coolant is getting into the cylinders. It doesnt happen till the car heats up cuz thats when the coolant is under pressure and leaking by/threw the head gasket. You can still have a bad head gasket even if the oil and coolant are ok, It just means no coolant is mixing with oil or the other way around. You wont really feel it during acceleration because its all getting blown out the exhaust so fast it doesnt have time to screw things up plus the leak may be small. I could be and HOPE im wrong. If it is the head gasket it sounds like its a small leak so far. AT least if it is bad it can be fixed in a day or 2.... well at least on a NA not sure of the 951

Pull the spark plugs and see what they look like. Also if you have the tool, test the cooling system by pressurizing it if it doesnt hold or it leaks down then you know its leaking