(T) Tempra 1.6 heat problems

Hello. I had this problem for a long time, but now it's getting hotter outside and the problem has gotten worse.
While going at 100+ km/h, when the car is heated to a normal temp (90), it (the temp) starts to jump. Slowly going up from 90 to 95(when the cooler starts to work) and than, with the help of the cooler it get's beck to 90 for about
+-30 seconds, than it goes again.
The rediator was replaced 8 month ago, it didnt help... the sealing on engine head (i'm not sure it's called like that in english... sorry) and the thermostat were replaced too... didn't help.
What could be the problem???

PS the problem is the same, only a bit worse, when going at low speeds...

PSS Sorry for my english...

waterpump?

I dont think so... i checked it and it works fine.

I understand that the temp might jump once in a while even if driving at high speeds, but not every minute?

really when travelling 30mph+ it should remain fairly constant generally. can't remember but is 90 in the middle of the temperature gauge? hmmmmm. only thing i can think of is possible slight headgasket problem, but then it'd probably overheat and not lower with the fan.

what about the heater fan in the car? does it blow out hot air all the time when the engine is warm?

could be an airlock in the heater matrix perhaps?

it does sound a bit strange though but i'm sure you'll be able to sort it out somehow.

The heater fan works normally, it blows out cold and hot air.
It could be a headgasket problem... it's old so it might have some tiny-tiny cracks in it.
But it's too expensive to buy a new one... at least until it starts to overheat... so i'll have to live with this problem for now i guess.
It's not serious enough for buying a new headgasket.

PS Yes, 90 is in the middle.