Have a 2.6 gas 1990 300E that if I use everyday it starts up just fine and runs well. If I let it sit for more than three (3) days It is almost impossible to start and when it finally does after cranking and cranking it runs ruff for a mile then everything is fine and if I start it everyday I'm okay again and it runs perfect. What should I check, bad fuel pump pressure, cold start valve, how do I check these and any other ideas. Thank You for taking the time to read and respond. Roger