E320 throttle lag

I have a new E320 4matic wagon (600 Miles) which has developed a low speed throttle lag. Driving in a parking lot at 2-3 mph, apply a little throttle and nothing happens. Sometimes it acts like it's going just stop running. My dealer tells me to switch to regular grade (87 Octane) fuel. Is he right, or just trying to avoid a repair? Problem shows up more it wet driving conditions. I've added 8 gallons of 87 to tank which had 92 in it. After a short 7 mile drive it seemed to be better. Is the vehicle equiped with a fly-by-wire throttle system that might need a sender unit or servo motor replaced?