Help on 3517

I'm trying to troubleshoot one last
flakey axis on my stepper driver board.
Comparing to the other working axis'
with clean square pulses, I think I've
found a signal on the 3517 Pin 14 that
looks like:

_ _ _ _ _
. . . . .
__ __ __ __ __

I don't know what the Acsii will look
like when you read it, or even much
about interpreting o'scopes, but the .
. . at half voltage and short duty
cycle is a dirty signal from somewhere,
isn't it? Throws off the half step /
disable / phase type thingie? Where
would it come from- I haven't added
shielding yet, but I'm still at 1Amp...
Swaping out the chips didn't change
anything; the pin in question is the Lb
second level output, which is 680ohm
from the Zero current half step position
indication output and 47k to Vcc with a
Cap to ground. What is the concept here-
that the Cap should filter out that
transient?
Or should I be looking at the input to
the 3517- perhaps the RC circuit to the
Mono flipflop on pin 12?