Alex,

When they don't work, do you try a few times and they still don't work, or just once or twice? Are there any other electrically operated anomalies?

The whole process starts in the fuse box and goes to the light switch and then to the key, I believe. I would start by cleaning off the fuse box connections as they are spring loaded and as they get crudded up, cooler, damper weather will make the connection there most susceptible to intermittent operation.

Since the light switch and starter switch are pretty much inside the car and protected, you might check some of those outside connections again. On my old 220D, when it was pretty new (< 2 years old) we took it to Aspen, Colorado on a skiing trip. The car became impossible to start unless we pushed it and I was getting no glow plug resonse. Turns out the wire to the plugs broke inside the insulating sleeve to the eye type connector and while it looked ok, it was not ok. I found it by yanking on the various electrical wires. The bad one broke off in the cold, I fixed it and everything was fine for about 20 years.

Your next bet is the light switch and next the starter switch. I believe the power to the starting switch, operated by the key, comes from the light switch. Over time, these connections can get worn and unreliable and the wires themselves can fatigue from becoming embrittled (usually due to getting pretty hot from use) and the typical Diesel vibration. In colder weather they can make poor or no contact, become more susceptible to fatigue, and moisture just adds to the problem (fatigue cracks become corrosion sites). Either switch can be at fault, as can the wires themselves, especially where they enter the connectors.

Good luck and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)