The card reads:

When Revealed: The first player draws this card into his hand. Ill Tidings cannot leave that player's hand. Then, if the first player has 5 or more cards in his hand, Ill Tidings gains surge.

http://www.cardgamedb.com/index.php/lotr/lord-of-the-rings-card-spoiler/_/the-voice-of-isengard/ill-tidings-voi

I am wondering how to play with this card: when (and how often) will 'Surge' trigger?

Just once 'when revealed' and you do have 5 or more cards in your hand.

Or

Every time your card count rises above 5?

In other words:

I assume that I cannot discard Ill Tidings even when playing with Erestor. But at the beginning of each round I will draw cards until I reach the 5 cards. (1 natural, 1 I.T. and three because of Erestor) Will Ill Tidings trigger each round then???

A "surge" effect only resolves when an encounter card is being revealed. So this only happens once for Ill Tidings.