Disconnect ground cable from batt, then get ready to remove starter . Usually lots of caked-on oil and road dirt to take 5 -10 minutes to clean off around starter mounting bolts and flanges When you have starter out check teeth on starter and on flywheel..You need a good light and soneone to slowly rotate the engine with key off and in Neutral. Could be every so often starter is engaging FW where it has some worn or damaged teeth.

While starter is out check if solenoid is working smooth and snappy everytime. Have motor body held firmly and use a good 12v batt. If necessary disassemble, clean and lightly lube solenoid, or replace it.
With solenoid working well the starter drive should shoot right out (to engage the FW) as the motor spins it rapidly w lots of torque.

Steve