Speaking as an electronics tech (since 1982) and having worked for JVCKenwood in R&D, I can tell you there are several possibles. None are economically fixed, new heads are $80-200. Labor is $60 to $120 an hour these days for anyone who knows what they are doing. And finding tech manuals and parts is about impossible.

1) The watchdog timer circuit for the uP fails, probably due to a "dried out" electrolytic capacitor in the watch dog timer itself. This means the little 'tickle pulse' at power up never happens.
2) Corrosion on a circuit board, that eventually eats away a path under an LSI or VLSI chip. This corrosion can come from condensation due to years of warming up and cooling down in extreme weather conditions, causing small amounts of condensation. Reactions due to TINY current flow thru this condensation makes things worse, but it takes time for this to show up - our cars are getting old, so the time has come.
3) Corrosion on pins that connect circuit boards, meaning that one board can't "talk" to another board.
4) Any number of other issues common in elderly electronics.
5) Damage to electronic components due to surges on the 12 volt input from weak battery, crappy alternator (poor regulation, severe ripple), severe spikes due to starter engagment or disengagement in the presence of weak battery or corroded battery cable clamps, bad/missing ground strap from chassis to block, etc. (This is why I did the mod James talked about, and I did it even to my Ranger.)

For $85, you can find way better heads than our outdated ones from the 1990s.