Heater works sometimes, but not really
 

My 2005 xb seems to warm up slowly.
After maybe 5-7 miles, I start to get a small amount of heat from the vents (blower works fine).
Heat will barely blow for 30 seconds and then proceed to get colder and colder until there's no heat..

Extra info---I just replaced my thermostat then my water pump (wasn't getting water flow and was overheating-all is well now)

After reading a few posts here, I'm thinking it may have something to do with the controls/linkage/damper setup inside the dash.

Any suggestions or ideas would be appreciated.
Even if it's just a link or where to look on this site to get more info.

Air bubble in cooling system? Remove radiator cap & run at 2500 rpm with heater on full blast, no A/C. Wouldn't hurt to replace the radiator cap as well.