Ok so now they checked the engine light that recently came on and I was told it was a SAI Secondary Air Injection Pump. I was assured it wasn't the Cat. But I'm still confused on how that would make the car jerk and hesitate upon acceleration. Anyone ever hear of this as the cause to my problem?

Hey guys,
I bought a used 2003 A6 3.0 from a gentleman that took real good care of the car. I test drove it on many occasions and he kept up on all the maintenance on the car. Paper work to show for it all from the dealer mechanic. The car ran like a champ until it started with the jerking and stuttering. It doesn't do it when its cold. I was told it was the engine mounts. So I replaced all of them. It helped only a little. It still jerks and stutters until you get past 40 mph. I have read a lot of opinions and theories. Even a clutch plate problem? Does any one know wtf the problem is? Is frustrating as hell. Ive never owned an Audi before and everyone told me not to get one because they're nothing but problems. Needless to say I had to have one. I tried to do my homework as best as possible to find a well maintained and loved Audi. But its appearing that I bought a lemon. ****HELP