Coolant system pressurising and leaking coolant from cap.

Basically if I drive the car fast the coolant system seems to build up so much pressure that the coolant starts coming out of the cap.

i took it to a local garage who done a pressure test on two seperate occasions couldnt find nothing.

he replaced the cap with a new one and I thought the problem was solved, I drove over 1000miles to france and back not a single drop of coolant leaked.

then it started happening again, changed the cap again and it was sorted for a couple of hundred miles and its doing it again, some people have suggested head gasket failure and that the exhaust gasses are entering the coolant system causing it to pressurize.

used one of those kits to test to see if there are exhaust gasses in the coolant and it come back negative.

there are no other signs of headgasket failure either, no oil in coolant, no water in oil, no sweet smelling exhaust fumes. Engine is not overheating there are no other problems at all.

i drained as much coolant out of the system as possible by disconnecting the bottom rad hose and replaced with fresh coolant and bleed the system, there is definetley no air locked.

anyone have any idea what could be causing this?

Golf MK5 1.9 TDI 05 plate