mystery losing coolant when driving under boost.

[popdemonic]

my coolant reservoir is losing 300-400CC when driving the car under high boost.
on normal driving,let's say cruising it or give it some boost.everything is fine.
i checked the coolant and motor oil.it seems there's no sign of motor oil
or coolant residued on both system.i doubt i lost some compression through out
the coolant system but the engine is running flawlessly,the coolant temp is
steady at 87C.i got the cpmpression test recent around 165psi on all 5.the engine
was taken from the early UrS4 with stock paper based headgasket.i got one
brand new multi layer headgasket laying but still waiting the RIGHT time to pull
the head off again.the engine was swapped on this Jan,2014.that's why i'm trying
not to bother it for awhile.Any easiest way to check on this.i have not done the pressure
test on the coolant system but i'm thinking everything should be fine(no visible leakage).

[Cottis]

You would probably see it but I had a tiny crack in the header tank on my car a while ago where the pipe to rad goes. It was bubbling out and I kept getting the fault beep and low water noise. I thought my head had gone and was getting worried but then it got worse and became visible. Changed the expansion tank for a new one and all was well. If yours is old, they do tend to fracture after a while. Hope it is something simple like this. If it is, buy an OEM tank. They are only 20quid or more than a copy and last a long time. My original one went for nearly 23yrs.

[popdemonic]

yes,i replaced new reservoir with genuine one on Jan,2014.