Intermittent Swimming Pool Leak

Odd leak story.
For the past several year or so the water level in my in-ground gunite pool will occassionally drop 3 inchs over night.
Sometimes this continues for a while.
Not worse during the day so I don't think this is related to the discharge side.
I've let the water level drop to see where it stops.
The water continues to drop below the skimmer so I don't think the problem is in the suction line.
I've packed the lamp fitting with epoxy and used dye at all the jets looking for a leak, etc.
I thought about using an ultrasonic dectector but the leak seems to stop by itself after a few days, making it hard to continue testing.
Any hint as to where my leak might be or how better to find it?
TIA.
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Hi,
Have you acid ewashed it lately? If so make sure you mix a lot of water in the bottom with some baking soda or soda ash as not to burn a hole in the sump casing on the main drain. Either way check the main drain. Shut the pumps off and release some red food coloring near the drain and see if it goes in there. Same method for other areas in suspect Hon. If it don't work pay $150 to have all lines pressure tested by a pro. Any pool supplier will be able to hook you up with one.
candice
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In alt.home.repair

My guess, it is related to the water table level. Does it start leaking after long dry spells? No visible cracks?
Get a mask on and check out your drain for the plug in the bottom. I forgot the name of it but it is to equalize pressure below the pool before you drain it to keep it from popping out of the ground. Is it loose or missing? Is the bottom cracked? Take your food color down there and gently put a drop in and see if there is flow if you can hold your breath long enough.
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