coolant loss,overheats on 2000 BMW 323i

started with plastic pipe under manifold busted.overheated.replaced pipes,heater hoses,thermostat,radiator,top hose,bottom hose,expansion tank,water pump,thermostat on expansion tank,flushed system,new bmw blue antifreeze.had block test for combustion gas-was neg.,took to bmw dealership and was told head gasket.OK here is mystery- no oil in anitfreeze,no antifreeze in oil,no smoke at all,runs perfect,heat gauge stays at perfect 12 o'clock until it looses water.another mystery- ran perfect last night,coolant level light did not come on and had water in expansion tank but soon as i took cap off today and vented at bleeder port water went down.apparently block was dry.i am not crazy yet but headed there.can it have a blown head gasket and still run as great as it does?? any info will be greatly appreciated.

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