Here's the situation:

I have a mediocre hand, but haven't made any aggressive moves in the hand (2nd pair, TPWK, etc.) and am out of position. The board doesnt look like it improved anyone by the time the river is out. Opp makes a large (usually pot sized) bet on the river after betting the flop and turn. My thinking is that this has to be either a very strong hand (ie set) or else it is a stone cold bluff with A high.

I feel that i may be folding too often in this situation. Not too many people are going to be making the large river bet with a hand with showdown value that isn't a lock to win.

This also holds for when the board gets scary (straight, flush, paired board, etc) on the river. Most non-nut hands will check behind on a scary board to get the free showdown that they feel they have a chance of winning. It is really only the certain hands that should be betting here (nuts/nadda).

Am I wrong in my thinking that I should be calling more in these spots. Currently I am folding about 90% of the time with 2nd pair/TPWK, but this seems like it isn't optimal.

Obviously this only applies to certain opponents, but it seems like it makes sense. please stop me before I do this if I am completely off.