I've seen quite a few rapid start F40 ballasts that either cause the lamps to flicker visibly, run the lamps dimmer than normal, or both. What about ballasts that have a flicker at startup but stop flickering after warming up for a few minutes? I have a ballast I removed from school that underdrives lamps and causes them to flicker continuously. What causes this behavior?

By the way, I don't mean ballasts that have a naturally low ballast factor running lamps dim. I'm talking about, for example, a .73A ballast that runs lamps dimmer than other identical ballasts.

Obviously it has something to do with age/wear, as well-used ballasts seem to be more susceptible to this.

Flicker at startup, disappear after few seconds: Normal with aged lamp, where the electrodes are asymmetrical (one heats faster due to lower mass, one ned higher temperature,...).
Flicker during warmup or all the time, lower brightness: Ballasting capacitor lost it's capacitance (usually the "selfrecovery" feature had to act too many times, each time lowering the capacitance by few 100's pF; quite normal capacitor wear, the initial breakdowns caused frequently by the TDDB mechanism). Less frequent with oil filled capacitors (but those contain PCB's), where the oil can move at least a bit - it prevents charge chains formation and even at breakdown, it clean up the area, so does not require to disconnect the section as necessary with solid dielectric capacitors

On most Magnetic Ballasts you will find flickering at start up.  how long the flickering lasts depends upon the temp in the environment. I use Magnatech magnetic F40T12 ballasts in one of my church buildings that is still T12 equipped and many of them will flicker for about 20-45 seconds, sometimes up to a minute when the ambient temperature is between 45 and 67F.  Above 67 I only see a flicker for 10-20 seconds at most.  And yes with some of the older ballasts the flickering is more consistent and longer lasting but after they have warmed up to operating temperature, the flickering dissapears.  Now, I use the 40 Watt T-12s.  If I use the 34 or 25 watt T-12s, they will flicker much more noticeably and much longer, until that lamp and ballast reach normal operating temperature.