98 Chevy express 2500, 8500 GVWR

The Van Stalls or acts like it's going to stall when I
pull up to a stop sign Or put it in gear . Just sitting
there at an idol It will build,  16 psi of vacuum.
When I put my foot on the brake it drops 1 pound
,Then when I put it in gear it will drop to pretty
steady to 9 pounds  . Once in a great while ,
something will give, It will  idol OK for a second
and  it will build-up to 12 psi and then idol bad and
drop back down To 9 pounds.  And then as soon as
I put it back in park the vacuum will go back up.
My fuel pressure gauge reads 58-59 psi idling and
then while I'm driving it Holds pretty steady around
63 - 65. Pretty darn close to what the old one did. I
have given it a tune up replaced the  throttle
position sensor, the idle air control valve , ERG
valve, removed and cleaned ERG pipe and port,  
upstream bank2 o2sensor, changed transmission
fluid and filter , fuel filter, compression test, all 180
except #6 170, and #8 190. Spark test was good .
As I'm sitting here letting it idle listening to the
Brewer game every 5 minutes or so, it will let a
light back pressure relief sound go from
underneath . I had just gotten this thing 2 weeks
ago still not running right the previous owner did
cut the cats out To clean them or punch them out I
am not sure I just noticed that There were clamps
on each side of each Catalytic converter .