Do a couple electrical checks first:
A healthy electrical system is essential to good cranking, especially with a hot motor. A deficient electrical system can have enough juice to crank a cold motor that's not up to clearance specs, has a tad less compression and "looser" clearances. ...but when things are heated up and more cranking amps are required, it just doesn't quite do the job. I'm not saying this is definitely your problem but the basics need to be checked first.
1. When hot, car off, nothing running, put a volt meter across the terminals
and check battery voltage. It should be 12.0 to 12.5 or so. Next, with car
hot, have someone crank the engine while you check the battery voltage.
It should be around 10.5 to 11 volts. Measured in the same fashion, your
charging volts at idle with nothing running should be 13.2 to 14.5 volts.
2. If the voltage readings above seem normal then you need to make sure
you're getting full battery voltage all the way to the starter. Loose
connections, inadequate wire sizes and defunct starter relays can cause
voltage drops. These voltage drops can cause exactly the problems you're
experiencing. This is the easiest way to check for a cranking voltage drop.
Get about a 6' piece of light gauge stranded wire and strip one end back
an inch or so. Disconnect the negative terminal on the battery. Put the
front of the car in the air, on jack stands. Crawl underneath and loosen the
starter positive terminal stud nut (noting the connection and how tight it
was in the first place). Wrap the stripped end of the 6' piece of stranded
wire around the starter stud and tighten the positive terminal back down.
Feed the stranded wire out from under the car. Drop the car and
reconnect the battery. While cranking the car read voltage from that
stranded wire to the positive side of the battery. You should read less than
1/2 volt. If you read more then you have connection problems, inadequate
wire size or a defective starter.
Note: This type of test should also be conducted on the ground side too,
done in a similar fashion. While you're under the car, ensure the
starter bolts are tight and the same on the engine to frame
grounding strap.
I'm running outta go juice so .....
Dave
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