ML55 issues

I have a 2000 ML 55 AMG, and I am at a loss. I am having issues with a check engine light and poor running. I have already replaced the fuel filter, fuel pump and MAF. Plugs were done less than a year ago. I am at a loss. it runs great for a day or 2, then lots of missing and hesitation then the check engine comes back on. My portable reader can't even read the fault now.
I am at a loos, any insight would be much appreciated.
Thanks,
jason



