Sounds like glow plugs should be OK, and fuel quality is likely OK from the tank. However, it may be that your injectors are not all "equal" and one may be either a bit dirty or otherwise not moving correctly when the injection pressure is reached. You might have them removed and checked. You used to be able to buy just the nozzle bits and pintel but I think you have to buy a fully assembled injector now. And they used to be about $80 as a fully assembled injector from MB....no idea how much they are now.

Jim

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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)