What is the ambient air temp and what weight oil is in it?

Does it have a block heater? Block heaters make a big difference. If equipped, check the cord condition before using, they deteriorate and can be a fire hazard.

Valves adjusted?

Yes the starter may be going bad, if replacing it use a Bosch 300D/5cyl version.

Measure the voltage drop/difference between what is in the battery while cranking vs. what reaches the starter to test for a high resistance positive cable. Check the ground to the engine, use a jumper cable from the battery to the engine to see if it helps.

Good luck!!!

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