I have a 98 XJ that in april of 2017 had the whole top half of the motor rebuilt new head gasket new water pump new thermostat had a valve job done as well since it had two cracked valves now this winter in the -20 degree weather it decided it didn't want to blow out hot air anymore it did something similar last year and I back flushed the heater core and it worked just fine up until now but when its in the above 0 temp say like 20 to 30 so once a month now here in ND it will blow out half ways warm air I think it may just need the heater core replaced or another back flush but I'm not sure it could be something else any suggestions would be greatly appreciated and yes the coolant level is fine thanks in advance

 

Flush the heater core both ways.

 

Double check that the coolant mix is good. It could ice up if it's super watery. It probably needs a good back-flush.
You don't have any coolant leaking on the passenger floor or abnormal fogging of the windsheild, right?