I have a 550 with 300 miles acting the same way started out as I was driving along it would sputter like it was running out of gas but if you floored it then it would go but I don't always want to be going 30 mph anyway it's progressively gotten worse and now it will start while pushing accelerater and only runs about 5 minutes. I'm very interested to see what you find out about yours. I guess I should add that I've checked the fuel pump (seems fine) changed fuel injector, fuel filter, tmap sensor and spark plug. Actually ran better with old parts