I'd say what you are experiencing is pretty typical of that tree, and it sounds like you already have a handle on one solution. Supplemental watering during the hotter/dryer months should decrease the problem (but add maintenance and cost of water). Be sure to check the soil moisture before irrigating. The tree can withstand brief periods of even waterlogged soil, but wouldn't do well in a perpetually wet area.

If you need info about transplanting, here is a link to my generic rootball size guide:

http://sites.google.com/site/tnarboretum/Home/generic-rootball-size-guide

And, here's a link to my tree and shrub planting/transplanting instructions:

http://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub

Please let me know if you have any questions or recommendations regarding the instructions.