Originally Posted by Marc sierra

It sounds like the head gasket has gone. Mixing oil and water only happens when it's leaking between the oil and water connections, but there is only one oil way through the head gasket and there are several water ways. So the chance of having oil and water mix in case of head gasket failure is not really that great.

The pressure relief valve is in the cap of the header tank and apparently it is working as it is sometimes pushing coolant out.

If it's not the head gasket it could still be that there is a lot of air in the cooling system. The DOHC cooling system is not that easy to bleed. There is a hose coming from the heater matrix going above the exhaust manifold, and because the hose runs very high, air can stay inside this hose. If you tie-rap the hose at a lower location the air is able to flow out.