Intermittent OVerheating

[Monooxide]

I have replaced the Thermostat and Radiator.

When I first replaced the thermostat...It would NOT open. What so ever. Now it seems to open.

The car will warm up to operating temps. and then overheat to "H" and then after maybe 10 seconds it will come back down to Operating Temps. If I am on the Highway it will NOT overheat unless I let off the throttle and slow from 75 to 65 to pace a slow car in fron of me. I have Altima Fans which are on 24/7. If I am in stop go traffic. Everytime I take off from a stop light it overheats then after 10 seconds it will go back down. Car is definately overheating, coolant is boiling in the resevoir sometimes. My heater is only warm on the Highway and that is on "4" and its only warm. On the streets, stop and go its cold. It never is HOT, its only warm.

Any Ideas? This intermittent overheating is rediculous.

Thanks,
Sean C.