Why would a 99 Malibu dump coolant on the ground after the engine's been turned off?

Three times now, my '99 Chevy Malibu (70,000 miles) has lost a good amount of coolant (nearly a gallon) AFTER the engine's been shut off. I only notice the puddle after returning outside. Twice, I've taken it to the dealership. After $1,600 in bills there (replace gaskets each time, once a water... show more Three times now, my '99 Chevy Malibu (70,000 miles) has lost a good amount of coolant (nearly a gallon) AFTER the engine's been shut off. I only notice the puddle after returning outside. Twice, I've taken it to the dealership. After $1,600 in bills there (replace gaskets each time, once a water pump, etc...), it did it again last nite while I was at dinner on a roadtrip. I put in DEX-Cool at a Walmart and kept an eye on it before leaving the city I was in. Satisfied, I drove home (200 miles). Well, this morning, all seems well: no coolant lost. I'm hesitant to take it to the dealer and get gouged for another $700+. Anyone have any ideas what could be causing this?

THANKS in anticipation!
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