Burble at 3-4K RPM

[Magnet21]

All;
I have an 80- CBX. 5800 miles since complete engine rebuild by Tim's. I chased what I thought was a contaminated fuel problem last fall. Bike started running crummy (lack of power, cutting out, hard starts) and I noticed little black particles in the external fuel filter. Figuring I could take care of that, I changed the filter with no improvement. Since all was pointing in the direction of a fuel contamination problem, I took it to my favorite shop and the carbs were removed cleaned and all had "kits" put in them (gaskets, etc.). Tech commented that the carbs looked clean and that none of the black crud found in the filter was found in the carbs.
After it was put back together, it ran better for about 50 or so miles and began cutting up again so I took it back to the shop. We eventually found that one lead on one of the 6 plug wires was not firing the plug. Dead coil...OK, now we are getting somewhere I thought. Coil was replaced and it ran well again. Plug check showed all 6 running tan to brownish tan.
It still will have some occasional "stumbling" around 3-4K RPM and is slow to warm up and will occasionally die at idle. Above 5K rpm, it will flatten your eyeballs with its acceleration and has no cutting out or stumbling all the way to 9K. Unfortunately, it is pretty much illegal to ride at those settings on public roads.....
I put some unleaded ethanol free racing gas in it and that helped some but did not get rid of the stumbling all together.
Where to look next, what did I miss?

Thanks to all that answer in advance.

[FalldownPhil]

What parts exactly were contained in the "kits" that were put into your carbs??
If the slide needles were changed from the stock ones, that could be your stumble.
Are you running a stock airbox and filter?
Best,
Phil

[Magnet21]

Stock airbox & filter. Love the intake roar on it! Unsure on slide needles, will have to find out.

[wyly]

it sounds somewhat similar to issue I had a month ago( but maybe not "if" your carbs were cleaned properly)...it turns out my small jets were dirty, it ran like crap up to 3-4000 rpm but great when I grabbed a fistful of throttle...I ran through two tanks of gas with Sea Foam and that brought back a healthy low speed carb response... trying Sea Foam 1st saved me the annoyance of tilting the engines...

a couple years ago I had a "professional" clean my carbs and he did less then stellar job( many of the small openings were plugged as was the air cut valve, he never took off the a/c valve cover), I had to clean them again myself before it would run properly...not every mechanic is capable of maintaining a cbx...