well, did you try the obvious and look for broken hoses?  did you look to
see what exactly is moving when this happens? did you put a vacuum gauge on
the line feeding the controls to see if the vac is really dropping off or if
it is something else?  if this is a 85.5 or later, did you check the
servos - they aren't vac operated but the standard broken little white clip
can cause problems.  Did you check that the heater valve is closing when
it's supposed to?  did you check the vac line routing and connections to be
sure they match the manual (you do have a manual, don't you?).  Have you
considered a tech that maybe has a clue if you can't do this yourself?
diagnosing this kind of thing isn't terribly hard - did you check the one
way valve in the vac line to see that it's working?  is the vac reservoir
ok?