> Some days I produce massive amounts of value, while others I barely get anything valuable done. I'd like a work place that would encourage me to go home and enjoy myself on the bad days and stay as long as I want on the good days, instead of expecting a regular 40 hour work week every week (I have flex, but on average...).

In the words of Stephen King, "Amateurs sit and wait for inspiration, the rest of us just get up and go to work."

Very often the value is in just showing up and doing the work. Even if the work isn't your best work, it's still better than no work. You can always fix bad work later. Even if you throw it away and start again, you've already primed yourself on the problem.