IMHO, the two most likely causes here are either bad fuel or something up with the electrical system. If catching up on maintenance and replacing the fuel isn't sufficient, the next thing I'd try is making sure that the battery has a proper charge and that the leads going to the battery are properly connected. While, you're at it, also make sure that the negative is properly connected to the frame.
I know that on my bike, I had a loose negative lead and it led to the bike bogging down and then moments later wanting to accelerate when the wire regained proper connection.
The good news is these should both be easily addressed without having to go on a deep dive into the engine.
I suppose, you might also have some moisture in the line, but that should also go away if you drain the tank and refill with new gas.