Buick heater problem



My son has a 1989 Buick Park Ave with 160K.

Has the electronic heater control and recently he said he is getting very little heat (some not much).

The car will die, then when started the heat will blow hard and the car will run like it’s brand new for 20 to 30 minutes. Heat will start to go away, car starts running rough and stay that way until it all starts over again.

I know this is not a lot of info, but he does not live with me so all I can do is relay what he told me.

Also, he just changed the thermostat in hope that would take care of the problem. Didn’t help.