Its either that or a short. It does sound suspiciously like mine when the regulator went. It didn't fail outright but just went partially insane. Sometimes it charged. Sometimes it sucked the life out of everything and you never knew when it was going to do which.
Yep. That's another possibility I forgot to mention. I have read of instances where the R/R fails after it warms up.
A relatively easy way to test for that condition is get the system warmed up by riding close to home, and then stop when you suspect it may be discharging and measure the chargnig voltage across the battery. Or just buy a cheap digital voltage monitor from ebay and tape it to your fairing to monitor the battery voltage as you ride.