300E 4matic runs rough when warm
 

My 300E 4matic has 150K has started stumbling at take off when it is at operating temperature. It does not die but misses. This lasts for just a second or so and then seems to run fine, even with acceleration. Plugs where replaced. Ignition wires are original. Could it be a fuel problem? It was running rough and I started using Shell gasoline and it started running much better after one tank(just like the TV ad says). Now after several tank fulls of Shell it is back to running rough or stumbling at the start. It also starts hard when warm, but not cold. Any ideas as to what to try? Always appreciate all the help from this forum of experts.

Of course you need to start with a complete ignition tune-up including cap, rotor, & wires.