1991 Holden Rodeo unusual air bubbles in overflow

Hi my Holden rodeo lately has been blowing air bubbles into the overflow. I first noticed it 1 afternoon after driving the car when I shut it off I could noticeably heard it bubble constantly for approx 5 seconds then settled down. After that I hear noises like air coming through the over flow when I am accelerating and more noticeably when going up or down hills. I have spoken to other mechanics and they all had there different opinions; one said that either the head is cracked or the gasket has blown, another 1 said it was the radiator cap wasn't sealing correctly and letting air get it in and another 1 thought that the radiator cap was the wrong 1 for my car and could be cause it to over pressurize. Well I have gotten another radiator cap that is correct according to the part list at the shop I got it from and it is still doing it. Also when I squeeze the return pipe to the radiator I can feel that there is bubbles there. I have checked that the oil hasn't gone milky and the car runs fine otherwise. Any advice would be great.

Thanks.

Nathaniel Kershaw