Backfire (afterfire?), shifting, and brake light issues

[Orphanclone]

After going over the thread from my last post a couple months ago, I was unsure of the future of my GZ250. I completed some easy repairs and took the bike to a reputable mechanic and have been riding around ever since. I've accumulated nearly 900 miles with no issues whatsoever (except for bugs smearing across my jacket & visor).

Until today.

I was out riding with a friend and he told me my brake light wasn't working. We stopped for gas and I figured that light not functioning from engaging the rear brake is an easy fix (tighten cable/shorten the connecting spring), but I am at a loss for what might be causing the same malfunction with the front brake. (Yes, the brake light still works. I can get it to turn on/off manually by moving the brake light switch.) Any thoughts?

We continued our ride, but I had my friend stay behind since he knew our route and had functioning brake lights on his GS500. Shortly after maintaining 50/55 for a few minutes, I noticed shifting seemed a little delayed, but chalked this up to possibly not disengaging the clutch entirely. (I have a tendency to cover the clutch, especially on roads I'm not familiar with.) I began to engine brake as we approached a stop, and heard a loud pop. I thought it was a backfire, but after reading the forums, I'm not so sure. After getting back up to speed with no issue, I noticed some hesitant shifting again when I downshifted for a turn, and this time I made sure that the clutch had been smoothly and fully disengaged. The backfire/afterfire occured twice more, both times when engine braking above 40 mph. At slower speeds, the engine seemed to run perfectly fine, though I did have a couple more shifting incidents.

A post-ride inspection revealed nothing. Fluid levels were where they should be, no detectable leaking, and nothing seemed mechanically out of place, so I'm stuck. I would really appreciate any input you might have.