And a mild overheating query

[jonboy]

 1998  Forester, just shy of 400000  kms, LPG

This is something I've not come across  before,  everythings generally fit  and running well except that suddenly, on my usual weekly round trip of 250kms  almost entirely on highway, the temperature gauge which I swear never moves beyond more or less halfway, once normal temp is reached, winter or summer, and we are 31º, 32º for he last month and in high summer mid afternoon maxx is usually high 30s and 42, 43 is not uncommon. , traffic, high speed, never budges... has started to creep slowly towards the red. after 3000 revs, 110 kph. The first time I saw it about a week ago i thought oh-oh, and dropped back down to 2500 revs with, and about 20 k to get to the motorway services. by then it gone back to 'normal'.   left it for about fifteen mins then checked  water level in the rad and in the plastic tank normal, no leaks ... carried on and for the rest of the trip, ,same thing , over 3000 gauge creeps slowly pretty close to the red, but slow down, and the old trick of opening the  heater and it goes back to normal   no leaks, levels good, no boiling over . oil level good  ....Failing themostat, ?  but that's usually the opposite, , water pump perhaps,?  time soon for draining and antifreeze renewal, as we can get -2 or -3º in february, some nights.. so shall I drain it all and do a rad flush ,  or is it an early warning...head gaskets,....?  Allthoughts and ideas welcome