All of a sudden, as of yesterday, apropos of apparently nothing, maybe
80% of starts result in a solid Check Engine light plus a steadily
blinking Shift Up light. The Check Engine light is on 100% of starts
now. When Shift Up is flashing, the idle is a regular stutter (not
like the variable stutter from a vacuum leak), and there is a regular
ticking noise coming from the vicinity of the #2 injector; open
throttle is sputtery (as opposed to more violent detonating/
backfiring), and power is noticeably reduced. As if one cylinder is
not being fueled. When only the C.E. light is on everything seems
normal, even over the course of the 500 miles I drove home earlier
today. The two conditions seem to vary mostly by start, not during
driving, although the Questionable condition has on one or two
occasions turned into the Bad condition within a few seconds of
starting.

Since there's no detonation and no smell of fuel, my best guesses are
that either the injector is failing, or less likely the computer is
crapping out in an idiosyncratic fashion. Before I spring for a new
set of injectors, is there any less expensive potential fix I'm
overlooking? What's perplexing me is why a fouled or failing injector
would vary per start but not while driving, unless there's something
analogous to a PC's POST routine going on.