[Whaler]

I wonder if your starter is getting hot and giving you a little grief?
The 10 minute cooldown does not seem like long enough to bring it down to temp but who knows. I would think that if it was a timing issue you would have these slow starts all the time.
A poor ground should also be apparent all the time. But do check that you have a good ground to engine strap.

If you could see how hot your starter feels...possible that the header is transferring heat. A shield is available that bolts right on.

Just a thought.

Tim