Sounds like a water problem - possible that the plant has really outgrown its container, which means there's too much root and not enough compost in the pot, so can't hold onto enough water for the plant. Other possibility is too much hot sun - they like a little shelter from midday sun.

More water required.
Root prune in spring before buds open.
Remove big thick roots to encourage fine feeder roots to developing.
Replant into original container. Place 2" of gravel in the bottom of the tub and then cover gravel with broken crock before adding the growing medium, this will ensure good drainage. Use a thin stick to push growing medium right down amongst the roots. Make sure there are no air pockets amongst the roots or the roots in this area will die back. Stake the tree if neccessary until feeder roots develop.
Root prune every four years or sooner if need be.
Do NOT feed the tree until one month after the leaves have opened.