Same problem here with my 2008 XL 1200 C. Right at "boulevard speed - rpm" where you describe mine stumbles or has a dead spot. Get it past there and she runs very good.
Took it back to the dealer where I bought it and they said "no problem - it's in your head". Took it to another dealer 50 miles away and it has been there 5 weeks. Problem is a mismatch in compression, the front cylinder is 20-25 psi lower than the rear. Harley (they refer it as the Motor Company) says the diff should be less than 5. Right now the camshafts and gears are being replaced under warranty. Will let you know if this works, maybe this week.
"Always choose the lesser of two weevils..."
Jack Aubrey, HMRN