It's a little odd but not impossible that all five plugs would fail at once. I could see it happening when the weather turns cold and five plugs that have been slowly going bad suddenly get worked harder and give up as a result. Valve adjustment is a good idea and helped my cold starting.

One other thing to look at is your engine oil. I'm running 5W40 in my 300 and 15W40 in my 240, and there is a noticable difference in cranking speed on cold mornings. Couple that with the difference in glow plug styles (loop vs pencil), and I get a 300 that starts on the first (ok first or second) try with a slightly longer than normal glow at 20 deg F, and a 240 that takes a 2-minute glow followed by a couple long cranking events to get going.

Before anybody says it, yes I know that 15W40 is only reccommended down to 23 degrees. I didn't think about that when it was in the 40's and I was doing my last set of oil changes.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.