Do you have exceedingly long warm-up times and does it take a long time to get the bike started?
Do you let the bike idle a lot?
To charge this little battery, you have to get the revs up a bit and you need to ride for more than little jaunts. It can take a lot of juice to turn the bike over on a cold morning and it's possible that with repeated short jaunts, you just aren't giving the battery time to fully recharge. The reason that you can go on long trips without a problem is because the stator (Also called the GEWTMIR) has plenty of time to charge the battery back up.
If the bike was cutting out randomly, or simply dying while at idle, I would suggest something else. But that doesn't seem to be the case, since you can complete the ride. But then you have problems starting the bike again after it has stopped.
I would recommend a permanently attached battery tender, if short trips are going to be common practice for you. It's good for the battery and you'll be certain not the have this problem one morning when you are already running late for work
Also, are you certain that you got the right battery for the bike? Even with short trips, this does seem pretty puny.