At 8 yrs., get a new battery (you'll need one soon enough anyway). Probably lacks the "juice" when cold to get the momentum going as quickly as when it was young. The hotter it is (up to a point) the more amps. it puts out and the less friction it has to overcome. The switch, solenoid, starter are all possibilities also, BUT....an eight year old battery? On a diesel? I'd go for the statisically probable here.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)