It probably leaks only when the cooling system is pressurized from heating up. You may be able to find exactly where the leak is coming from by pressurizing the system with shop air (15-20 psi), if that's available to you.

Could you elaborate more on "the motor gets pretty hot".

(Only 9 more miles to the 100,000 club!)

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95 E320 Cabriolet, 159K