something doesn't sound right....I can get mine to wheelie in any gear, even at 60+mph, with ease...granted my motor is far from stock, thanks to Pinnsonault, Pro Circuit, CrankWorks....but when I bought it in 2011, it was stock, and sacked out, and would still wheelie with ease in any gear.
as Motorrad mentioned...maybe you're losing traction...easy enough to determine, just look back over your shoulder whilst you're on the gas.
your clutch isn't slipping is it?
how bad is your cylinder?...you mention getting it re-plated in the winter...have you done a compression test to find out where it's at?