My problem is lower cadence ~65 rpm is the pain in my knees. Just like G105 said, when my cadence is 90 +/-5 rpm, the power level feels right, stamina maintains longer, and best of all- no pain in the knees. Heartrate monitor also tells me that I'm more efficient at the upper cadence levels.

But without a cyclocomputer with cadence I would have never figured this out.