Overheating temperature in a 3S-FE Options

[Titiko]

Hi guys, i get a 3S-FE in my celica, since many weeks ago i have noticed the following situation: during day by day drivin in the city the temp is Ok it achieve its operating value and it doesnt oscillate, but in weekends when i travel to a near city (about 1hrs of travel) when i have driven many miles @ average speed 90-110mph the temperature begins to oscillate, it behavior is very weird cause suddenly rise at 3/4 of the stock temp gauge and then drop to the middle of the gauge (std temp value) and it happens continuosly but each time it rises higher and higher. I dont know why it happens cause in the city the temp always is OK. I replace the old thermostat, i removed the radiator and it was cleanded with special fluid, the fans works fine, the timing is set @ 14degrees at idle, and then of checking all these things the overheating problem continues.

What do you think about this situation? what can i do? I think it coudl be the camshaft gasket, probably it get burnt.