Lack of heat in '99 Chevy Silverado

I have basically no heat in my 1999 Chevy Silverado, I’ve changed the thermostat and flushed the heater core. This helped slightly. Went from no heat to after long time little heat. Temp. guage is at proper area but still little or no heat…

And you have bled the system and ensured that the radiator is full as per the repair manual instructions?

Yes. I have heard such things(from click/clack etc) that there are doors/motors that can be stuck. Or some kind of heater relay selinoid? Sometimes there will be some heat for 10-20 seconds then blows cold again.

What temperature thermostat did you use? Most modern EFI systems require a 195 deg F or hotter thermostat. I don’t know if that is the problem, but it’s a thought.