all good suggestions.
a search here under funola should turn up a thread on testing starter spin speeds.
also amp draws, and glow plug testing.
there is also a great DIY on rebuilding the starter.
it sounds like you have some valves out of adjustment, and you are using DINO oil.
synthetic oil flows better at cold temps, and is easier for the starter to push during cold spells.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!