Playing two pairs is supposed to be a strong play on the flop, but I'm finding a problem with my play when it comes to playing pocket pairs with a board pair.

Most books and sites recommend playing 2 pair strongly, but they don't differentiate between the two types of two pairs.  I've been burned enough with the board pair/pocket pair combination to begin to question if they should be talked about seperately, or even in three categories - overpair in the pocket with a board pair, underpair in the pocket with a board pair, and the "hidden pair", which I still tend to play very aggressively.

Anyone else have advice with this conundrum?