Dakota240sx

hello everyone i have a 93 hatch and it started overheating and was blowing white smoke out exhaust and had loss of power. so i changed the head-gasket and sent the head out to be checked and everything. got it back and completely redid all gasket and everything. then after i got everything back together it runs great again and no smoke.

until two weeks later ill be driving down the road about a mile or less and it will heat up two notches below the red but after 10 seconds it will go right back down to where it should be. it will be completely fine until i slow down then it will go back up. but if i rev it up then it will slowly go down (sometimes). also i changed thermostat, didn't help. thanks in advance