w124 misfire

[E420 93']

Hello, i have 93' E420 and he doesn't work like he's suppose to do, and I was wondering if someone could give me answers!
The problem is, he has misfire and I changed recently sparking plug in the engine, because the old one was wet of gas and dirty!
nothing changed. still has this misfire, so it isn't the spark plug,
this happened for about year ago and I changed sparking plug and then the engine ran fine! until now, we have checked the spark-plug gap and there came only spark on one of two spark-plug gap,
he gullet before I changed spark plugs but that's not problem now!

he always starts and runs like he's suppose to do until you give in a little gas, then the engine turns off and the engine give away bad sound and acting like he doesn't work on 8 cyl..

(sorry about the english, i'm from iceland and i'm not very good speakin english)