Good grief, you have a good one going there.

Must be fuel/air related.  Air filter okay? Could be a plugged gas tank
vent system.  Try driving with the gas cap off for 30 miles.  The low
and pulsing vacuum at idle shows a leak somewhere.  It should be a
steady 15 to 22 inches at idle.  Most likely a bad hose.  There is a 1"
diameter short breather hose that goes from the crankcase breather to
the air intake hose that could be plugged or collapsed.

Check all hoses including the big air intake hose to the throttle body,
they tend to crack in the ribs.  Use the propane method (carefully) to
check for leaks.

Be sure you can see light when you look through the cat (remove it from
the muffler), it sure sounds like a plugged cat.

Good luck, and be sure to let us know what you find.

Stuart