My Kingdom for anyone who can solve overheat problem I’ve had for years!

For years I’ve had an ongoing overheat problem that miraculously cures itself but returns to haunt me again roughly once a month.

The temp needle generally sits at halfway (12 o’clock) position but sometimes it creeps close to the overheat and sits there for a while before eventually returning to the normal 12 o’clock position.  However, sometimes it swings fully over into the overheat and I instantly stop while it gurgles and cools.  Strangely no overflow water visible.  On these occasions the electric fan doesn’t kick in as the bottom hose is cold so doesn’t trip the fan sensor located on this hose.  I’ve changed the radiator, hoses, thermostat (many times), water pump and still the problem periodically persists!

When overheating I notice the expansion reservoir is full to the brim. But once cooled it returns to normal level with no water lost in the system!  I can then drive off and everything is fine for the next few hundred miles!  Can’t be leaking/blown head gasket as engine has run fine for years with no white smoke, spitting water/oil from exhaust.  No foam/oil in coolant.  When coolant changed no gunge in old water.

Any ideas on this persistent fault that can’t seem to be fixed?  Chris