So I have a 1999 jeep cherokee(xj) and this last summer I replaced the heater core, radiator, and water pump, but now that it is starting to get cold my heater doesn't want to work like it should. So.etimes it works fine other times it will get warm while the jeep is warming up and then start to cool down as you drive and other times it doesn't work at all, I've flushed my heater core a couple time in the last month to try and flush anything out and it would work good after I did that for a day or two then go back to not working, just looking for any ideas of how to fix it or any suggestions, thanks.

What does the coolant temperature gauge read when you see these variations? Does it match up with the symptoms?

Cab heat is a product of engine heat so the first suggestion is to verify coolant temperature. Get your mitts on a point and shoot laser thermometer (very inexpensive now) and point it directly at the thermostat housing of a hot engine. Then you'll know for sure.

When you did cooling system work, did you replace the thermostat and radiator cap? Critical components of your cooling system. Your heater core is probably fine as you replaced it a year ago unless your cooling system was super dirty and that dirt made it into your core.

Good luck and keep us updated!

Doublecheck the coolant level to make sure its not too low as that can cause inconstistent heater temps. Low coolant means more air, which can get trapped in the heater core. Once that air escapes, the heat comes back.