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When i first bought my car i did not have any problems with engine temperatures. Then after a couple of hundred kilometers i did a general service after that i noticed the engine temperature would rise very quickly and then drop to normal operating temperature, first i thought this was just a bit of air in the system. The longer i had the car the more frequently it happened. I checked all the normal things to no avail. In the end i was convinced it was a cylinder head. Removed both heads only to find this was not the problem and hoped that replacing the head gaskets would cure the problem because i was probably wrong about my judgement. After the first couple of trips i was a happy chappy no heating issues. ( this was also during the hot period of summer!) It was now cold weather and yep the same symptoms occurred. I was now convinced that a liner had dropped. Not happy with the original cooling system i bought a Subaru Radiator and plumed it in with a different expansion tank. This was the preparation for an bigger V8 from a competition safari Land rover which has tuned for extra torque. The car ran very well with the original engine, i was thinking yeah it's ok now but it will play up. I have a Land rover which i have also extensively modified, i wanted the Range rover expansion tank from the Mantula for the Landie. After fitting the expansion tank in the Land rover low and behold the symptoms began to appear in the Landie, after careful inspection of the expansion tank i saw that the top of the screw thread from the filler neck was filed off skew wiff, because it was to high when fitted in the Mantula and fouled the bonnet. I felt like a right berk after all that time and effort only to find out it was the expansion tank. The original engine only has 19000 km's
Off the top off my head (without going into the garage to check) there are two types of thermostat which you can use 73 deg (early Range rover) and 82 deg opening temperature. I have used both, the lower temperature seems to work better when in traffic jams etc but when on the move i think that the engine runs too cold so i prefer the higher temperature thermostat and rely on my electric fans to do their work.
I would not run any engine without a thermostat unless i was desperate. (it could cause more damage)
Have you checked that the system is fully de-aerated of has enough coolant (sorry if this sounds stupid but mostly it is something simple)