Not sure. Does your cap have a seal inside it? Does the cap thread area look like there are cracks or abnormalities? Seems to me that you get heat that causes this problem and that probably means heat is expanding something and causing leaks, but by the time you try to find it it has cooled down and you can’t see it anymore. I’m betting you have a hairline fracture someplace in your threads or the top of the tank that you can’t see unless it’s been heated up at which point it expands and coolant escapes. Interesting problem to have though quite annoying.

 

The first time it happened I found that some plonker had cross threaded the cap - it has very fine threads on it, so I replaced it as soon as I could but that didn't fix it and as I said, although it leaks when using the car it doesn't overheat or go through much coolant, its weird all right.
Maybe there's a problem with the new cap, they aren't that expensive and I love wasting money on this car.
I need to change the waterpump and coolant anyway so I may as well take the tank to a radiator shop and have it pressure tested.

 

Did you figure out the cause? My 04 Gallardo has similar issue.

 

Cap was redesigned for 2008. I can get you the newer cap if you want to try that first. cell/text 717-319-2100

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