Connect the starter directly to the battery with some jumper leads
If it cranks properly then you have a wiring problem .
Usually it will be the ground strap corroded through under the insulation ,.
Check by running a jumper from the battery - to a good grounding point near the starter then hit the key
Engine cranks properly = bad ground
Also remember the ground is via the engine so f the engine mounting bolts are loose the ground will be poor .