Heater goes cold unpredictably.

Most likely just low on antifreeze. You may need to flush the system, especially the heater core. If you are handy, its not to hard. Plenty of videos on youtube . Try just adding some coolant to the reservoir tank first though. There should be full hot and full cold marks on the tank to help get the correct level. If you are a bit low, watch it for a while afterward to make sure you aren't losing it regularly, could mean more problems.

Thanks! I'll check the level.

99percent sure low coolent,need to be burped and more coolent added pull it on a up hill grade and run it and add more coolent.sometimes if you hold the rpms at about 2500 it will clear itself too but id say that your problem because i just replaced radiator and had same problem of no heat at idle one she got air out no more problem ,good luck.