MBWorld.org Forums - CEL & Misfiring problem E320 1999

I am new in this site and appreciate your help.
I have a 1999 E320 with 160k miles. Recently, I tried to accelerate hard going uphill, when the engine started to sputter like only 3 cylinders working with not much power and CEL came on. Stopped the engine and started it again and the sputtering went away but CEL stayed on. I could reproduce it by pressing gas hard to high RPM especially uphill. Took it to a mechanic and he said the cylinder 1, 2, 3, & sometimes 5 are misfiring and could be fixed by changing spark plug and wires. He asked for a lot of money and I decided to do it with my son.
I changed wires and spark plugs and air filter. Engine started beautifully and sounded a lot better than before. However, try to accelerate on uphill same thing happened. Easy acceleration is no problem but hard acceleration put the engine in sputter mode with very little power until I stop and start the engine again. Do you have any suggestion?