I have 2000 C230 4 cylinder model with 50,000K miles. Just recently it has been starting hard. I have spent over $400 for new plugs, fuel system flush and filter. I have also spent over $500 for a new mass air flow sensor. It now starts fine after previously running but still starts a little hard when starting after it has been sitting for a while like overnight. The mass air flow sensor was the only code reading under the diagnostic which is why it was replaced. The technician's seem stumped. Any ideas?