Is the wall unit set to 24 and then the wall unit resets to 20? if this is the case it sounds like you need to certified person to look, you might be able to get something fixed under warranty.

... know that I'm not a Refrigeration guy so take with a grain of salt... If the ice buildup is around your coils , you'd see the buildup on the side and back , here is what I would do. 1. Switch the unit to Cool mode (2) Turn temp down to 18 (3) turn fan on the lowest setting. Keep a close eye on things, this should melt the ice buildup. Once the ice melts ... switch all settings back to where you had them. If the ice is around your fan located on the inside of unit... this would be a first for me and I've never problem solved it before. A couple obvious things come to mind, but I wouldn't suggest or recommend on here unless I tried it before. Hopefully your issue is the first scenario... Good Luck, hope this helps !

I know this question is a year old, but I would think that you might want to clean the unit(s) out, and filters need to be clean.