90 maxima heating

Hello, my wife uses a 90 maxima for work and it's been a great car. The problem is that recently it has been overheating, not right away but if you drive it for about 20 minutes the gauge starts to raise almost to the max.
I first changed the thermostate and that didn't make a change. Yesterday I changed out the water pump (what a job) and it still does the same thing.
I can't get any heat from it at all, and I know the thermostate is working because I tested it before installing it. If you let the car run with the rad cap off it will blow the coolant into the air after warming up.
Does anyone know what this could be? Thank you in advance Ivan.

Hi my wife drove the car to work and it overheated this morning. Went and pulled the thermostat and it ran good all the way home. Did not overheat, but no heat.
Is it the stat or something else ?