Cooling problem

[hgilbert]

Since fitting a new engine to my Magnum I am experiencing a slight cooling problem.
Engine is a 383 SBC, thermostat 83 degree (with two 4mm holes) and temp guage Auotmech mechanical.
Original set up was with a CBS header tank and a top hose filler. Both the header tank overfow and the top hose filler overflow were connected to an expansion tank.
On start up the water temp would rise normally and then suddenly shoot up to 120 degrees falling back to the mid 90's almost as quickly. This would happen two or three times before settling down to run in the low 90's.
Letting the engine cool and removing the header tank cap indicates a vacuum in the system but the header tank and top hose contain no water.
Last weekend I took the top hose filler out and rised the header tank as high as possible as well as raising the expansion tank. I've bled the air out of the system by slackening off the water temp sender unit but the problem still persists. I've also changed the thermostat.
The large (bottom) outlet from the header tank goes to the rad bottom hose and the smaller top outlet goes to the top of the rad, similar I think to the pilgrim set up.
The obvious explanation is air in the system but if that was the case I would have thought that the temp would not settle to normal.
Any ideas gratefully received.
Harry

[hgilbert]

I think my thread (above) failed to register for some reason.
Harry

[Clarkson]

I remove the top coolant pipe from the thermostat housing and remove the thermostat from the intake manifold. Raise the top coolant pipe High and Fill the cooling system till the water just over flows out the thermostat, that way all the air escapes from the block and heads. Refit the the top pipe and it should be all good.