When I lost my pump I wasn't able to get a new thermostat at the time I replaced it and after that I had a problem like yours. It would take the needle a while to actually get up to the "N" and then it really wouldn't go any higher unless I was idling for a long time in hot weather. What happened is that my thermostat had been damaged by the abnormally high coolant temp when the pump broke. The thermostat had failed in the open position and coolant was always circulating throughout the whole system even when it didn't need to.
How sure are you that the thermostat was actually replaced?
