Have machined the head,put in a new gascet,new radiator,waterpump and thermostat. still overheats, but not always.

If I'm reading you right I think you have a plugged radiator, drain some coolant out then look at the tubes in the radiator after you remove the cap, if they are plugged you should be able to see it. Also check to make sure the belt is tight.

Intake manifold gasket may have been installed backwards, allowing coolant flow to the heater core but not to the thermostat.

Disconnect the positive battery terminal, then the negative. Let it set for a few minutes, most recommend 10 minutes. Once the time is up, reconnect the negative, then the positive battery terminals. This should clear the computer's memory, which would solve that issue...if it's the computer that needs to be reset...

Always change the thermostat after every time it over heats.
And while you are at it, you should probably do a cooling system pressure test, to make sure you don't have a bad head gasket.

Could be the head gascet. It is not totaly dead, but some channels are damaged. Because of that, some pressure from detonation chambers are getting in to the cooling sistem channels, trough damaged head gascet.

Open coolant reservoir cap, then start the engine.

Put the cap on for ten seconds and take it of.

If there is presure building up or the coolant is bubling, than thats a head gascet for shure. Just replace it and dont forget to chek the head for micro cracs.

Sounds like a blown head gasket .But thats easy bad relay switch or coolant temp switch or bad resistor.

If you are doing them all at the same time, do the water pump first. I like to do the toughest part first and then move on the the easier parts. Then the thermostat and then the radiator. Once you drain the radiator most of the engine coolant will come out making the waterpump easier to clean up. If you are doing them in different day's, alway's replace the waterpump and thermostat in pairs. If you can do the radiator at the same time, I would suggest that. Sometimes when thermostats get out of fluid, they sometimes will lock up and cause and overheat. Then you are replacing all the fluid and thermostat on a hot engine.

Only 2 parts remaining are the thermostat and waterpump. Always start with the thermostat when diagnosing an overheating problem. If the car is overheating constantly under all conditions and coolant is backing up into the reservoir, it's very likely to be the thermostat. If it only overheats at low speed or idling, it's more likely the waterpump.

Check the availability of morte than one thermostat for your engine. You may have the wrong one in and it's not opening at the lowest setting. Causing the engine to overheat before it will allow the coolant to circulate properly. The other answer lies within the efficiency of your waterpump.

Good point above. Does the fan come on?

If you have no heat, you have air in the heater core.

You say the head gasket is ok because yoou have no oil in the water. Many times the head gaqsket blows combustion into the cooling system only. Get the car up to temp with the radiator cap off, put it in gear, hold the brake and load the engine. If you have a lot of bubblls out of the radiator, you have a blow heade gasket or cracked head.

Check the ignitin timing. Old BMW's would overheat in a heart beat with retarded Ignition timing.

My bet is head gasket because the heater core is air bound

good luck