sounds like ur thermo is bad or going bad.....it should reach normal hieght (sp?) after about 5-10mins or whatever dep. on the weather.....

it should not rise or drop once the normal temp has been reached which is about half way or so, give or take (up or a down a litte), i'd replace ur thermo and see if that helps...

about $30 form honda dealer, and 2 bolts to unscrew, fairly easy