Mana and Rage Regeneration Question

[BluePhoenix]

I am playing a Mage Amelie (level 16), and so far, the class is pretty fun.

Now, I have Transmute (1/3) and Concentration (1/3). I have noticed that in the long fights- for example where I have to kite a stack of 30 Paladins for 28 rounds- the mana and rage regeneration gets lower and one time stops altogether.

At the beginning of the battle, I get rage from my troops attacking enemies and enemies attacking me, and I get the normal mana from Concentration. But as the battle goes on (specially in longer battles), the regeneration gets lower. I get 1 mana from Concentration instead of 2 and I get no rage. And then after some more rounds, I get no mana and no rage.

Can someone please explain why this happens? Does mana and Rage regeneration gets lower with rounds? Or does it get lower with the less number of enemy troops?

I am using the Gift Bag mod.