If you have one of those Halloween smoke machines, fill the intake with smoke and see where it comes out.

I have an 03 roadstar 1600 that is doing the exact same thing. Itll go through the gears just fine getting on the highway but when i try to maintain speed it sputters and runs like crap. Did you ever get yours fixed?

Everyone has the carb covered how about loose throttle cables...................or perhaps the little screw thing that ya tighten them too, I'm guessing this is rather new issure. I now with my old 77 I had all the weird issures but then again AMF's were alien bikes............have a great one! "T"

Sounds to me like it is starving for gas or has an air leak at the manifold. The reason it is only doing it when you are maintaining speed is you are actually compensating for it while getting up to speed. You probably don't realize it but you are most likely using more throttle than normal while coming up to speed... compensating for the low fuel to air mixture. When you back off to maintain speed is when you notice it.

With the bike in neutral, try holding it at around 2500 rmp and it will most likely spit and sputter.

I would first check for an air leak on the intake manifold (where the carb connects to the motor). If that's not it... time for a carb re-build.

IMHO I might add whenevr a "rejet" is done there is a big difference in the different jets manufacturers make,with my bike, people have had no luck using dynojet kits,.. the factory pro kits out perform them,this is given advice on a Honda board I belong to.Also like others have said check for leaks again,sometime those rubber boots can get tiny pinholes in them.
The air fuel mix screw only applies to idle & low throttle speeds,n I dont believe thats the problem.Are you using the choke a lot to start it up..on my bike its not good.Found it fouled the plugs n made it run poorly at speed.

I had a roadstar midnight star edition with hypercharger and and use to clean air filter with WD-40 (let it sit for a little on K&N filter)and gently scrub and rinse with garden hose. air blow from inner out with air line hose gently. check throttle control below airfilter and adjust appropriatly if a thorough filter cleaning isnt done. Never had a problem with that bike in the 11 years owning one changed fluids and cleaned filter smae set up as yopu and the bike ran great except i had straight pipes.