boiling after 3 miles but...

When I notice the temp is rising there's water squirting out bigtime & that's when I stop & refil with about 5-6 liters & then it's alright. So yep: there's water under the car + a track behind too.

The thing I really wonder about is why it's alright after the refill?
It's as if the cap don't let out the warm air & then it overheats fast in this hot weather, 25-30 C / 77-86 F.