Buddy,
I can empathize. I recently hooked what looked to be a 17-18 inch brown that I worked for about an hour - I'm stubborn, and this would have been my largest fish to date. Unlike every other brown in that stream, he refused to spook and continued to feed, so I just couldn't give up. Finally, he sipped a #14 Adams as if he didn't have a care in the world, and after about 10-15 seconds of working him toward the shore, he threw the hook. Needless to say, I was pretty disappointed.
With regard to larger than average browns in reasonably small streams, I've been reading a lot on night fishing lately, and while I haven't had any luck with it, I'll post some of the things that I've read - I'm inserting a disclaimer right now that I am NOT an expert, but I read a lot about this stuff to try to prepare for trips and whatnot. I would appreciate it if anyone who knows more from experience or otherwise would add to and / or correct me.
Large browns tend to forage feed at night. Especially with smaller streams, large browns have a hard time supporting themselves by drift feeding on small insects all day, so they need to go out and hunt for larger prey, e.g. bait fish, leeches, mice, crayfish, etc. A lot of these critters are more active in the evenings, so therefore, these browns are forced to feed accordingly. From what I understand, peak feeding times are oriented around midnight and just before sunrise, and the darker the better, i.e. the less moonlight, the better. I'm assuming that any time there is prey, however, the trout will be more active.
As far as fishing for large browns, I've read that you should fish big, black flies close to the surface, so that a nice silhouette forms against the night sky - Lloyd Gonzales' book has a nice bait fish imitation that fits the bill very well. I can see this advice as valid, but in my opinion, there's no reason why a woolly bugger (or the appropriate crayfish pattern) fished near the bottom wouldn't work as well. Regardless, I'm guessing that color isn't as important though. Basically, "loud" flies like woolly buggers or hackled wet flies are a good bet. I've also read that a lot of the darker-toned wet flies tied in larger sizes will also work, e.g. #6 Royal Coachman. The stonefly nymph you drifted would likely fall in the category of a possible night-time fly.
The other bit of info. that I found interesting was that these browns tend to move from the deeper holes, where they reside during the day, to shallower water at night. In fact, I've heard that at times, they move to water shallow enough to where their dorsal fins stick out of the water. I can actually testify to this, as on a recent outing, I spooked a nice sized brown out of shallow water when I know that earlier in the day, he had been residing in a deeper pool not far from there. Shortly after this spooking, I was nosing around and noticed a number of crayfish and bait fish hanging around that I hadn't seen earlier in the day, either.
I'm not saying the only way to catch large browns in small streams is to fish at night, but it's information that I found interesting, nonetheless! Like you, I had been doing a lot of my fishing in the AM or afternoon, but rarely in the evenings. After reading what I have, I have adjusted my fishing times to include evenings every so often. Hope this is useful, and again, if anyone wants to add to and / or subtract from what I've written, I would be more than happy to read what is written! Thanks a lot.
Chris.