If you wait until the dahlia starts to wilt before watering, you are continually stressing out the plant. Sounds like either you need to water daily, or plant directly into the ground and keep on the same watering schedule.

Also, it is important to water until you see it leaking from the bottom. The fertilizer you are using will act like a poison if water never drains out. I've rotted lots of tubers by being frugal with the water when fertilizer is in the pot. Make sure you allow it to leak out and leave the bottom instead of being reabsorbed into the pot.