It has been awhile but basicly the most common Feeling is when getting on it hard the bike feels like it cuts out till you let off the throttle.
There has been discussion on advrider about it You will have to use google and search as I don't remember all the details.
Or does your bike stay smooth but just run out of power??
Could the clutch be slipping ? Do the rpms go up when it happens ?
Maybe the fuel injectors are not flowing enough ?
ST1300 - IRON BUTT # 23,716