MY 99 buick is having problems with the distribution of the heated air to the vents, on the driver side the air flow is hot and weak, but on the passenger side the air flow is strong but cold, also on the display of the heater and airconditoning control panel, the temperature figure flashes. Is it the vacuum lines, or is it the control panel ??????

Have same problem with 2000 Lesabre Custom. A friend suggested that there are actuator valves(electrical I think) that control the vent valves under the dash. They have to be checked and/or replaced.

Had a similar problem with my Park Avenue last December, turned out the passenger side heater blend door actuator was bad.

How expensive a fix is that? Is it a DIY thing?

Read the following 2 threads. Could be a blend door, but also could be low refrigerant.

https://www.gmforum.com/mechanical-134/climate-control-diagnostic-88-93-a-285427/

https://www.gmforum.com/mechanical-134/default-redirect-cooled-air-late-model-gm-cars-96-2000-a-279092/

How expensive depends on which climate control system you have (Manual or Automatic). From what I've read, Manual has one temp actuator on the heater box behind the glove box. Automatic has two temp actuators, one on either side of the heater box.

What exactly do these actuators do? Are they expensive?

They actuate a door inside the HVAC duct that determines air flow. I don't think they are overly expensive. The main concern is usually getting to them, since they are buried in the IP.