Cold air blowing. Help!!!!

I have a 2000 Jeep limited XJ just about to hit 138,000 and all of the sudden over the past month once the Jeep is warmed up to about 185-190 degrees i turn the heater on and it blows hot for about 10 seconds then starts getting cooler and only blows about lukewarm. If I turn the heater off and back on it will do the same thing and blow hot for the first 10 seconds or so then get cooler and stay at about a lukewarm temperature. I am in North Atlanta and it has been in the upper 20's to 30's here recently. I had the water pump changed about two weeks ago and today changed the thermostat and still encountering this issue. Any ideas or suggestions. I have owned this jeep for almost 6 years and it has been garage kept the last 4 years, prior to that it was parked in a driveway, with little to no trees near it, so do not believe it has leaves clogging anything.

Thanks in advance.

 

Flush flush flush or heater core.

Its hot at first cause the heater core is heat soaked then you turn the fan on and the cool air hits the core and cools it down and can't warm back up.