Mark
I recently had the OXS and ALT lights come on and stay on in my '86 Westy.  It
happened after I had the instrument panel off  to access the hydraulics
underneath.
I really don't know why they came on but decided it had to be connections.  I
remove the panel again and cleaned all the contacts I could find in there.
Even unscrewed some of the nuts holding down the printed circuits and cleaned
them.   The problem went away.

You might check the operation of your idle stabilizer valve to account for the
idle problems.  You should be able to hear/feel it hum with the ignition on.
You could also crank the engine to idle and cut on the AC.  If the engine dies
or almost dies, suspect the idle stabilizer valve.  Also, turn the steering
wheel to its full extents while idling and see if the result is the same.

I have had my idle stab valve unplugged for a long time due to a fried ISCU.
I just live with the crappy cold idle.

Dan Hall
'86 Westy
Alabama