E36 1998 Model overheating

Hi Folks

My car is driving me bonkers and so is my garage. Here is the history car started to overheat so stopped as my last E36 did the same thing, cooled down and had car taken to garage.
The water pump had gone as well as the viscous fan thingie so that was replaced, the mechanic said there was slight preasure at the top of the head and it might be the overhead gasket. Now the car still overheats and when you remove the cap from the expansion tank it takes around 15 mins for all the gurgling and hissing and bubbling to subside, apparently it has been blead twice.

Now my mechanic says it is bubbling and hissing because chamber 3 and 4 has blowen, now what I am not getting is that is runs very well and the fault is intermittent there is no lack in power and the oil looks clean. On start up there is a slight ticking noise that goes away when the engine heats up,also the temp guage is at half way after 5 mins and the engine block is cold.

I have also read other interesting facts on the internet that the E36 is a bitch to bleed and that distilled water and only BMW Coolant should be used as they are known for overheating issues.

I just don't understand how it can be the head gasket when it runs so well please help