This bug is occurring to me as well. Max mana rising by 30 usually by opening some chest/shrine. I have version 1.6b7 patched with Russian 1.6.4. After several chests in dwarfish lands my paladin had ridiculous 170 mana. This really spoils the challenge of the game
You can edit your amount of mana with cheat console (Shift+~) by typing MANA 0 (to erase it to 0/0 mana) and then MANA 79 to put it back to 79. This is what I do, when I notice inexplicable mana raise by 30.