Mark,

Curious how long on the ground you are running the engine and at what rpm (max or idle or other) is the engine shutting down? Cooling holes work when the plane is flyng not on the ground. More details about the ground running scenario would be helpful in diagnosing your issue.

I live and fly in Houston TX which in the spring, summer and early fall has comparable conditions to Trinidad in air temp and humidity. Ask Mark Mendonca, do you know him? My typical cooling cutouts are similar to yours, no foam in chin cowl. Just wondering if you are asking the engine to run on the ground with the chin cowl on for a unreasonable period of time. Have you flown the plane to see how the engine performs while it's properly cooled?