erratic idle and running rich... massive vacuum leak?

We have a 300 that has a bipolar disorder.  It either runs great or it idles high, low and
dies and it is running rich - like 10 mpg rich...  I replaced the air idle valve and it was better
for a while but never perfect and now is up to its old tricks.  I also bought new hoses for
the connections to manifold and engine but have yet to put the engine side one because
the engine side is buried.  If you accelerated down a off ramp - 9 times out of 10 it runs so
rich you,d swear you were driving a hopped up diesel.  Also is there a large rubber gasket
in the intake that is prone to cracking.  Thanks.