I'm with you. '91 300E. I have done the same and everything seems to improve then it goes back to the same. If I turn the key before starting and prime it 4 times, it will start strong. Have you tried that? Supposedly if it does, you don't have enough residual fuel pressure. I've seen a lot about replacing the EHA valve too. Everyone seems to have this same ongoing issue. I'm perplexed. Wondering if a stronger battery with more cranking amps will help.