I would try a new cap first since it is probably not very expensive.  I have a
feeling however that you have a head gasket leak.  This is an internal leak
that allows combustion gases from the engine into the cooling system.  I hope
this is not the case, but if it is then when you change the cap to a new one
you will notice no difference it will do the same thing.
Let me know what happens.
Ken Wilford
John 3:16