Alot of pressure in cooling system

[colinR32]

So i have a B3 audi 80 1.8 auto(slow i know, but i like her)
I have done a head gasket on her a few months back along with new radiator and thermostat. Now when sitting in traffic or anywhere to that matter for a little
Period of time the pressure in the cooling system is unreal and causing coolant pipes to burst/pop off, now this only happens when stationary as for every other aspect of the car is perfect runs and drives spot on, alot of real warm heat from heaters ect but this pressure is my only problem, anyone any ideas??

Regards colin