For the most part, I'd guess it's nearing the limitations of the main jet.  The only thing that gives me pause is that you say it sometimes has power and other times doesn't.  That part makes me think of two things.  The first is interference with fuel flow.  This could be anything from a dirty carb to a leaky vacuum line to a clogged petc0ck filter.  Try draining the float bowl to see if it has crud in it.  Verify the integrity of the vacuum line and clean the petc0ck.  When you pull the petc0ck, you can check your tank for crud too.  In the middle of this, I'd install an inline fuel filter to help ensure cleaner fuel for the carb.  They run anywhere from about $2 to $45, depending on what you want.  I use a $3 disposable clear one and change it annually.  The second thing that comes to mind is less likely but possible anyway - clutch slippage.  This is not very likely with a low mileage bike unless it's been abused a lot or maybe you're using a pure synthetic oil not made for wet clutches or one that has friction modifiers that make it incompatible with wet clutches.  If it's generally just running out of oomph as you approach full throttle, I'd say it's time to rejet and retune.  I'm sure there are other ideas out there.  Anyone else?

at 55 to 60 mph you should have lots of power and acceration left. you should be able to jump up to 75 or 80 with just a twist of the throttle. even with stock jets. you have a carberator problem or something stuck in a fuel line.