Could be root rot.
Punch extra holes in the sides of that pot for improved drainage/air flow and monitor what times of day you see wilt. Does placing a dome over the plant cause it to perk back up again?

You MUST let the soil become rather dry between waterings or you will see wilt symptoms from not enough oxygen getting to the root zone.

Edit: That last plant actually doesn't look sick to me. Is it FLOPPY (doesn't look it) or just have a general shape that makes you think it's wilted?