Just some random ideas that may not have occurred to you:

1) Not enough gas in mix: possibly fuel filter due for a change;

2) Too much air in mix: possible vacuum leak--test lines and connectors;

3) Have you tried a thinner oil (i.e. 5W-40)? It's been a rough winter!

__________________
Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)