My 1992 BMW K100RS runs like a top, except below 3000 RPMs it "pops & hiccups" if not under full throttle. During hot weather, if I slow down without giving it any throttle, when I come to a stop, it backfires loudly just once. Above that RPM it runs perfectly.

I have run injector cleaner fluid through it & it helped some. I have not checked the color of the plugs.

Any ideas?

You probably have an air leak in the intake plumbing.  A plug reading after running for 5 minutes at low rpm would give a clue.