Power loss, 2001 Honda civic after about 30 minutes driving on hot days?

Strange power loss in 2001 Honda civic after about 30 minutes driving on hot days. After power failure car will only move at about 15mph with high RPM, but is fine once it cools off. During power failure the engine sounds ok. Occasionally its power will fail completely, the car will shudder slightly, stop and all the lights will come on briefly, and if this is the case it can be difficult to start. It has also been losing water with more being lost on hot days, I can’t really find the leak so could be head gasket maybe. I would be very grateful for and suggestions.

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