A chain with this many miles on it almost certainly has a large amount of what's called "chain stretch". As chains wear, the distance in between the link gaps increases. (This page on chain maintenance will tell you more than you need to know on the subject.) When this gets past a certain point, the chain needs to be replaced. If this isn't done, increased wear to the drivetrain sprockets almost certainly will result, since the chain no longer fits over the sprockets properly. (Since this is bike has an IGH, this problem will still exist but to a lesser degree, due to the lack of derailer gears.)
I highly recommend you not only replace your chain but also have the casette and chainring examined; they may need to be replaced as well. Once this is done, you'll notice the bike shifts more smoothly and works better in general.