CEL on b/c of missfire

[00ml320]

OK, I have been fighting this problem for a while now. I have a 2000 ml320 .The check engine light keeps coming on with the code for number 4 and number 5 cylinder missfire and a multiple random missfire. When i turn the light off it stays off for several days or until full throttle or rough driving. I have changed the plugs couple of months ago but i put bosch platinums +2. I think that the wires are still original ones. The car has 90K on the clock. Please help I don't know what to do any more. thanks in advance.