The bubbling is just the coolant boiling, because either the formulation is incorrect (not the right mix - nominally 50/50), the pressure is too low, or the temperature is too high. If your gauge is correct, and you have been adding lost coolant with the correct mix, then the new pressure cap should fix it.
After you put on the new cap, wiggle it. If it wiggles at all, it is NOT on completely! I found I was only comfortable pushing down hard enough on the cap if I first supported the tank from underneath with my other hand.
BTW, see if your old cap wiggles - maybe it is just incorrectly installed! If so, check for a bad gasket before putting it on properly.