1. could be the ovp. check part number of your ovp vs part number of newer design of the ovp.
2. Check distributor cap and rotor - they could be very worn.
3. when was the last time the plugs were changed?
4. have you run a fuel injector cleaner? Techron is usually whats recommended
5. have you had the coil's output checked?
6. for the fuel door, just push on the side closest to the front, and it will go in.
Let us know how it goes.
BTW, I have similar problem, but cold starting is fine. warm starts are my problem. I am just living with it, but I know my dist cap and rotor are in need of replacement. thats next on my list.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.