check your brake system ...

I'm confused by your description. If the ABS system was having a failure --  
speed sensor going out, valve body sticking, those sorts of things -- then
the lighth should come on and remain on through the completion of that
particular drive cycle. On the next drive cycle, the light might not come
on, but if it does then it ought to remain on until the key is switched OFF.

The system undergoes a self-test when you start the car, and the light
should come on for a few seconds during this test, then go out and remain
out. Any failure of the system renders the ABS unreliable, and the light
comes on to tell you that the anti-lock feature has been disabled. Since
there are no further self test cycles until the next time the ignition turns
on, the ABS light should never turn off (once it comes on during any given
drive cycle) until you park the car and shut it down.

Given the fact that you have an '88 model car (20+ years old), I suspect you
are having a grounding problem where the light can come on intermittantly
without regard to the actual operatioinal condition of the ABS system.

Are you sure the ABS light is the one that is on? There are other braking
system lights, one of them tells you the fluid level is low. Have you looked
into the master cylinder to be sure the fulid level is up?

When the ABS light is on, the only affect you will notice is that the tires
can lock in an emergency stop -- you can skid if you are not careful. If you
can live with this, then forget about the light. Your brakes work fine,
assuming they work fine anyway. There is no danger that you will lose brakes
altogether.

The bad news is that there isn't really much in the way of diagnostic
hardware for these things.

The good news is that things that blink when you hit a bump are invariably
loose connections.

I would start by inspecting all the cabling from the sensors to the ABS
box for damage.  Then I'd measure the static resistance of each of the
sensors and see if one of them is different.

You can't just plug a scanner into the ABS unit on these, and see plots
of the sensor inputs, the way you can on the newer cars.  That's a minor
annoyance.  Remember, 1988 is only the second year they sold ABS, at least
here in the US.
--scott

Thanks, I haven't had time to look at this. I thought I'd mention that
since it warmed up 10 degrees I haven't seen the problem. It's a minor
annoyance but I thought it might be a common aging problem with a
known solution. Thanks again.