ML320 hard to start - Looking for suggestions

Hi, I have a minor problem. Any advise would be greatly appreciated. My ML320 has been generally problem free. I rescently took it on a 400 mile trip. Half way through the trip I filled-up on gas. Later, I noticed that it was getting hard to start:

Normal was:
-Turn the key and it starts in 3-4 seconds.

Hard to start is:
-Turn the key and let it turn-over for about 8 seconds, doesn't start.
-Try again for another 8 seconds, doesn't start.
-Try again and it starts, runs rough for about 3-4 seconds, then works fine.

After running for a while (10 miles or so):
- Shut off engine and restart within 1 minute, starts normally.
- Shut off engine and restart 10 minutes later, it's hard to start.


I've run 2-3 tanks of gas through it since the trip, but have'nt seen any improvement. Any suggestions (dry gas, fuel filter, other)? Thanks for the help,
Don

Check out the fuel filter, on the "original" ML's (pre-initial facelift) there is a fuel filter modification (costing about £300!!!). In theory this shouldn't make a difference but it's always worth checking it out.