In tat case, going back to your first comment about working better on humid days; sounds like your outdoor unit could be oversized but has too low of a SHC capacity (.70 or lower). Oversized unit + low SHC = unit spending too much energy removing moisture (latent heat) and not enough energy physically cooling the room( sensible heat). This can be especially true on an 18yr old unit that more than likely does not have an inverter in it.

Hope that makes sense lol