So, although I'm sure you've checked already, might be a good idea to check one last time before moving on...
If there's NO pests then answer me these ?'s

Where did the wilting start? Bottom to top? Top to bottom? With the new growth or the old? Are the leaves curling up ( and if they've been yellow for a while did they curl up from the beginning or just recently)? A lot of time really yellow leaves with green veins indicate Iron deficiency, but if the leaves are yellow all over it could just be Nitrogen def. (dudn't seem likely with your nutes atm though), and most likely magnesium like you were thinking IF the whole leaf is yellow and curled from the beginning.

Oh and Magnesium problems generally start with old growth, while Iron def. affects new growth. Nitrogen def. generally moves up the plant from the base...
Manganese problems usually show themselves as yellowing veins, but the tips of the leaves would stay green and would not be as likely to fall of, so I'm ruling this out at the moment.
Hope this helps, let me know :P

edit: oh and a pic of the leaf/plant would be helpful as well, if you can.