318 "P" Reg Over heating

Can anyone help me please. I have had my 318 (E36) for 6 yrs and in the last 6 months I have had problems with over heating. The problem still has not been resolved, 4 garages have not been able to sort it out....£100's of pounds have been spent. This is a very long saga, but I will give you the bullet points.

On the first occasion a second hand raditor was fitted...over heated again within a week
Major damage was caused (my fault) and I took the decesion to buy a salvage engine.
After a few weeks it over heated again (towed home again) new thermosat fitted all checked and working
Again over heated after 4 weeks, water pump propeller had broken, replaced, tested ok
2 wks later again same problem (towed home) Fully tested and found no problems, not losing water. I have run the car on short trips and the temp gauge reached norm (middle) When I check the top hose this is very hot, but the hose conected to the thermosat is still cold!!! So even with another engine the problem is still there, new waterpump & thermosat..could it be the Raditor or do you know better. I feel I have lose all confidence and I won't take it on a long run. Any help in resolving this issue would be great.

Air in the cooling system. Drain down and refill it slowly with the radiator bleed screw (blue plastic screw about 15mm in diameter on the top right of the radiator viewed from the front of the car) open about 3-4 turns and the engine running. When you have a steady flow of coolant from the bleed screw, tighten it up and replace the radiator cap and run for 20 minutes or so. Loosen the bleed screw again and raise revs to 2000. Keep an eye on the bleed screw for more air escaping. When you've got all the air out, switch off, top up the coolant and nip the bleed screw up. Do not overtighten. There is another bleed screw on the thermostat housing which you can use if you have a particularly stubborn airlock, though personally I prefer not to as it places my delicate hands in uncomfortably close proximity to the fan blades.