Is the radio the only thing with a mind of its own?  If so, it could be
a simple short in the radio power supply, maybe the amplifier mounted
under the steering column.

If other things are doing it also, a usual suspect is the ignition
switch. I had the radio, wipers and A/C all doing it and when I
replaced my ignition switch it fixed it.

One way to test it is to wiggle the ignition switch, or turn it back
just a hair when you start the engine, but not enough to kill the
engine.

Also, my electric antenna is shot, so I think I'll just put in a
standard (old fashioned) hard-mounted one.

G'luck

If it's basically the same radio as the '89 240, then the most likely cause
is a circuit board inside the radio that plugs into an edge connector. Over
years of vibration, the connection becomes weak causing the intermittent
operation. I've done temp fixes on mine a couple times, but eventually it
goes out again. I would suggest just replacing the radio with something
newer to stop the headaches.