Video vertical synch too fast?

[bolesd@cortez.its.rpi.edu (David B. Boles)]

I use several Apple II - to - Apple IIe machines to run real-time labs
in human experimental psychology.  Recently the three oldest computers
have all developed the same problem, a vertical synch signal that seems
to be running twice as fast as it should be.  I have testing software
that measures the number of transitions made by the signal in 1 sec, 
and the number is reading 120 instead of the normal 60.  There is also
a test of the Mountain Clock Card against the video vertical synch signal,
and when an interval is measured using the synch signal, the number of
milliseconds on the clock is only half what it should be.

Has anyone seen similar symptoms in aging machines, and if so, what is the
fix?  If not, what should I try first when I start swapping parts with
correctly functioning computers?  It's not the clock cards, by the way
-- swapping in one from a functioning computer does not fix the problem.
Any suggestions would be much appreciated.

Dave Boles (bolesd@rpi.edu)