So, I ride for about 40min and end up with a flat battery.

I've changed the regulator, and watching the battery meter it maintains a steady 14.6v or so, +13v at idle until about 40mins in, then the voltage slowly drops to 12v.

If I stop, the bike will stall out and won't restart - I can bump start it and it will run a little longer, but the battery is by now dead.

I am thinking bad battery given the voltage is good throughout - anyone got an alternate suggestion?

 

No alternative suggestion, but I'd get the battery checked. Possibly a bad cell.