I think I might have figured out what happened with my attempt at overvolting the Krypton star. The culprit is the fact that standard flashlight bulbs have short lifetimes around 25 hours or so. Compare that to quality MR-16s, MR-11s etc., which have lifetimes of 3000-5000 hours. Now, a say 20% overvoltage of the latter gives an acceptable lifetime of say 300-500 hrs, but do the same with the flashlight bulb, and one gets a measly 2.5 hrs.

Of course, bulb quality varies from individual bulb to bulb. So I'm guessing this is why my slightly undervolted magnum star is still working after two years.

Now, the interesting question is WHY do the MR lamps last so much longer than the PR-based bulbs.