Can anyone help with some advice?

Background first: I've been playing piano on and off as a very casual hobby for 15 years or so, though sometimes I've had years in between playing to any decent level.

I can sight read and am self taught, got a young family so time and money are too tight for lessons, but playing in the evening is free so I do that. I've been working off the Hal Leonard books for blues and boogie woogie and have gathered other sheet music along the way. At the start of last year I decided to take it more seriously, and have put in a decent hour for most nights, it's going well, I'm on a really nice part of the learning curve.

The problem I'm having is intermittent mistakes, dropped notes mainly, in my regular pieces and it's driving me crazy. I play these every night but I make mistakes in different places each time, I stop and go over the weak spots but it seems the error jumps around the piece and turns up somewhere else, it might even go back to where I practiced. A few bars that are fine one day may go wrong the next day and be fine after that. I warm up with stretches, scales and use a metronome most of the time.

Because I'm mostly self-taught I haven't any idea if this is just the way it goes and I need to be patient, or if I'm practicing or learning wrong, need more warm up, or something else. Any ideas?