1998 Buick Century heat

It sound like a low coolant flow throw the heater core

Engine cool
lower the coolant level
Locate the two hoses running to the heater core
disconnect them at the engine side leaving hoses connected to heater core throw the firewall
use garden water hose and compressed air if possible
run the water and follow by air throw one side and out the other and vice versa
do that two to three times watching what leaving the heater core you are looking for the good flow and clear water to run throw
good luck
let me know

It start to overheat with no or little heat on the inside
u release pressure and then you get heat
if so
you were running with low coolant
let it cool off and check coolant add as needed
you have to open air bleeders